From 
PAUL    B.    HOEBER 

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Ovum  of  about  eight  weeks,  with  membranes.  In  the  upper  figure  (actual 
size),  in  the  upper  right-hand  area,  the  chorion  frondosum  is  apparent.  In  tlie 
lower  figure  (slightly  larger  than  actual  size)  the  membranes  are  laid  ojjcn. 


Fetus  of  third  month,  with  fetal  membranes  and  newly-formed  placenta  (nat- 
ural size).  In  the  upper  figure  the  chorion  is  cut,  showing  the  fetus  through  the 
amnion ;  in  the  lower  figure  the  membranes  are  laid  open. 


A  TEXT-BOOK 


EMBRYOLOGY 


FOR  STUDENTS   OF  MEDICINE 


BY 

JOHN  CLEMENT  HEISLER,  M.D. 

Professor  of  Anatomy  in  the  Medico-Chirurgical  College, 
Philadelphia 


WITH  J96  ILLUSTRATIONS,  32  OF  THEM  EST  COLORS 


Secont)  Bt)ition,  XTborougbli^  IReviseD 


PHILADELPHIA  AND  LONDON 

W.  B,  SAUNDERS    &    COMPANY 

1902 


Copyright,  1899,  1901,  by  W.  B.  SAUNDERS  &  COMPANY. 


Reprinted  September,  1902. 


tLECTROTYPED  BY  PRESS    OF 

WESTCOTT  a  THOMSON,   PHILADA.  «•     B.    SAUNDERS    8.    COMPANY. 


PREFACE  TO  THE  SECOND  EDITION. 


In  the  presentation  of  the  second  edition  of  this  book  the 
effort  has  been  made  not  only  to  make  such  additions  and 
alterations  as  the  progress  of  the  science  rendered  necessary, 
but  to  correct  whatever  errors  existed  in  the  first  edition,  and 
to  improve  the  work  generally  as  much  as  might  be.  For 
kindly  suggestions  along  these  lines  the  author  would  express 
his  thanks  to  the  various  reviewers  who  have  from  time  to 
time  noticed  the  work  ;  he  Avould  also  express  his  profound 
appreciation  of  the  generous  spirit  in  which  the  book  has 
been  received. 

The  publication  of  the  monograph  of  Peters  upon  the  em- 
bedding of  the  ovum  and  the  formation  of  the  placenta/ 
followed  by  the  work  of  Webster,^  whose  researches  confirm 
the  results  of  Peters'  investigations,  practically  setting  aside, 
as  it  does,  the  previous  views  upon  these  processes,  neces- 
sitated radical  changes  in  the  text.  Therefore  the  chapter 
treating  of  the  Deciduse  and  the  Placenta  has  been  rewritten, 
as  has  also  the  greater  part  of  the  section  upon  the  Chorion. 

In  addition  to  these  changes,  several  new  illustrations 
have  been  added. 

J.  C.  H. 

August  15,  1901. 

^  Die  Einhettung  des  Menschlichen  Eies.     H.  Peters.    Vienna,  1899. 
^  Human  Placentation.     J.  Clarence  Webster.     Chicago,  1901. 


Digitized  by  tine  Internet  Arciiive 

in  2010  witii  funding  fronT 

Open  Knowledge  Commons  (for  the  Medical  Heritage  Library  project) 


http://www.archive.org/details/textbookofembryoOOheis 


PREFACE 


The  facts  of  embryology  having  acquired  in  recent  years 
such  great  interest  in  connection  with  the  teaching  and  with 
the  proper  comprehension  of  human  anatomy,  it  is  of  first 
importance  to  the  student  of  medicine  that  a  concise  and  yet 
sufficiently  full  text-book  upon  the  subject  be  available.  It 
was  with  the  aim  of  presenting  such  a  book  that  this  volume 
was  written,  the  author,  in  his  experience  as  a  teacher  of 
anatomy,  having  been  impressed  with  the  fact  that  students 
were  seriously  handicapped  in  their  study  of  the  subject  of 
embryology  by  the  lack  of  a  text-book  full  enough  to  be 
intelligible,  and  yet  without  that  minuteness  of  detail  which 
characterizes  the  larger  treatises,  and  which  so  often  serves 
only  to  confuse  and  discourage  the  beginner. 

In  the  arrangement  of  the  subject-matter  of  the  book,  it 
has  been  the  aim  not  only  to  present  a  connected  story  of 
human  development,  but  also  to  make  each  chapter  as  nearly 
as  possible  complete  in  itself,  for  the  sake  of  convenience  of 
reference.  It  is  for  this  reason  that  some  repetitions  occur 
in  the  text.  The  frequent  allusions  to  certain  facts  of  com- 
parative embryology  are  rendered  necessary  by  the  very 
nature  of  the  subject,  but  it  has  been  the  writer's  aim  to  make 
these  allusions  as  simple  and  as  easily  intelligible  as  possible. 

In  the  selection  of  the  illustrations,  great  care  has  been 
exercised  to  employ  those  of  the  greatest  teaching  value,  and 
to  arrange  them,  with  reference  to  any  one  chapter,  as  nearly 


10  PREFACE. 

as  possible  in  proper  chronological  sequence.  Due  acknowl- 
edgement is  made  in  each  case  for  every  illustration  borrowed 
from  other  works. 

With  few  exceptions,  no  attempt  has  been  made  to  cite 
authorities  in  the  text,  and  the  author  would  here  express 
his  obligations  to  the  writings  of  His,  O.  Hertwig,  Kdlliker, 
Schultze,  Bonnet,  Balfour,  Marshall,  Piersol,  Minot,  Tour- 

neux,  and  many  others. 

J.  C.  H. 
3705  P0WE1.TON  Ave., 
Philadelphia. 


CONTENTS. 


CHAPTER  I. 
The  Male  and  Female  Sexual  Elements ;  Maturation ;  Ovu= 

lation  ;  Menstruation ;  Fertilization 17 

The  Spermatozoon 20 

The  Ovum     22 

The  Hen's  Egg 25 

Oogenesis 27 

Maturation  of  the  Ovum     30 

Ovulation     - .32 

Menstruation 35 

The  Relation  of  Menstruation  to  Ovulation  and  Conception     .    .  37 

Fertilization 38 

Artificial  Fertilization 40 

CHAPTER  II. 
The  Segmentation  of  the  Ovum  and  Formation  of  the  Blas= 

todermic  Vesicle .  41 

Segmentation 41 

The  Stage  of  the  Blastula 45 

CHAPTER  III. 

The  Qerm=layers  and  the  Primitive  Streak 47 

The  Stage  of  the  Gastrula 47 

The  Embryonal  Area 50 

The  Primitive  Streak 51 

The  Development  of  the  Mesoderm 54 

The  Derivatives  of  the  Germ-layers 58 

CHAPTER  IV. 
The  Beginning  Differentiation  of  the  Embryo ;  the  Neural 

Canal ;  The  Chorda  Dorsalis  ;  the  Mesoblastic  Somites  61 

The  Neural  or  Medullary  Canal 62 

The  Notochord  or  Chorda  Dorsalis 65 

The  Neurenteric  Canal      66 

The  Somites  or  Primitive  Segments 67 

11 


12  CONTENTS. 

CHAPTER  V.  PAGE 
The  Formation  of  the  Body=wall,  of  the  Intestinal  Canal, 

and  of  the  Fetal  Membranes 71 

The  Formatiox  of  the  Body-wall  and  of  the  Intestinal 

Canal  of  the  Embryo 71 

The  Amnion      74 

The  Yolk-sac .  78 

The  Allantois 80 

The  Chorion 82 

CHAPTER  VI. 
The    Deciduse    and   the    Embedding    of    the    Ovum ;    the 

Placenta;  the  Umbilical  Cord 85 

The  Decxdu^  and  the  Embedding  of  the  Ovum 85 

The  Placenta 88 

The  Ujibilical  Cord 92 

Relations  of  the  Fetal  Membranes  at  Birth 94 

CHAPTER  VII. 

The  Further  Development  of  the  External  Form  of  the  Body  95 

The  Stage  of  the  Ovum 96 

The  Stage  of  the  Embryo 96 

The  Visceral  Arches  and  Clefts 100 

The  Stage  of  the  Fetus 107 

CHAPTER  VIII. 
The  Development  of  the  Connective  Tissues  of  the  Body, 

and  of  the  Lymphatic  System 113 

The  Connective  Tissues 113 

The  Development  of  the  Ly'mphatic  System 116 

CHAPTER  IX. 

The  Development  of  the  Face  and  the  Mouth  Cavity  ....  118 

The  Evolution  of  the  Face 118 

The  Mouth 122 

The  Teeth 125 

The  Salivary  Glands      131 

The  Tongue 131 

The  Nose 133 

CHAPTER  X. 

The  Development  of  the  Vascular  System 135 

The  Vitelline  Circulation  and  the  Origin  of  the  Blood  135 

The  Development  of  the  Heart 138 

The  Metamorphosis  of  the  Single  into  the  Double  Heart  ....  142 

The  Valves  of  the  Heart 144 


CONTENTS.  13 

PAGE 

The  Alt.antoic  and  the  Pi^acental  Circulation 148 

The  Fetal  Arterial  System 149 

The  Fetal  Venous  System 153 

The  Formation  of  the  Pericardium,  the  Pleura,  and  the 

Diaphragm 158 

The  Portal  Circulation        161 

The  Final  Stage  of  the  Fetal  Vascular  System    ....  165 

CHAPTER  XI. 

The  Development  of  the  Digestive  System 168 

The  Mouth 175 

The  Pharynx 176 

The  Tonsil 177 

The  Anus 178 

The  Differentiation  of  the  Alimentary  Canal  into  Sep- 
arate Regions 180 

Increase  in  Length  and  Further  Subdivision .  184 

Alteration  in  the  Relative  Position  of  Parts  and  Further  Devel- 
opment          185 

Histological  Alterations 188 

Meckel's  Diverticulum 190 

The  Development  of  the  Liver 190 

The  Developbient  of  the  Pancreas        194 

The  Development  of  the  Spleen 194 

The  Evolution  of  the  Peritoneum 195 

CHAPTER  XII. 
The  Development  of  the  Respiratory  System 204 

The  Thyroid  and  Thymus  Bodies 207 

CHAPTER  XIIL 

The  Development  of  the  Qenito=urinary  System 212 

The  Development  of  the  Kidney  and  the  Ureter     ...  212 

The  Mesonephros  or  WolfSan  Body 213 

The  Metanephros  or  Permanent  Kidney 217 

The  Suprarenal  Bodies 218 

The  Development  of  the  Internal  Generative  Organs  .  220 

The  Indifferent  Type 220 

The  Male  Type 222 

The  Female  Type 226 

The  Bladder  and  the  Prostate  Gland 232 

The  External  Organs  of  Generation 234 

The  Female  External  Genitals 236 

The  Male  External  Genitals 237 

Summary 240 


14  CONTENTS. 

CHAPTER  XIV.  PAGE 

The  Development  of  the  Skin  and  its  Appendages    .   .       .   .  245 

The  Skin •    • 245 

The  Appendages  of  the  Skin 247 

The  Nails 247 

The  Hair      248 

The  Sebaceous  and  Sweat  Glands 250 

The  Mammary  Gland 251 


CHAPTEE   XV. 

The  Development  of  the  Nervous  System 254 

The  Development  of  the  Spinal,  Cokd 257 

The  Development  op  the  Brain 262 

The  Fifth  Brain-vesicle 264 

The  Hind-brain  Vesicle ."....  268 

The  Mid-brain  Vesicle 270 

The  Inter-brain  Vesicle 272 

The  Fore-brain  Vesicle 278 

The  Development  of  the  Peeiphekal  Nervous  System     .  292 

The  Development  of  the  Sympathetic  System 300 


CHAPTEE  XVI. 

The  Development  of  the  Sense  Organs 302 

The  Development  of  the  Eye 302 

The  Eetina  and  Optic  Nerve 304 

The  Crystalline  Lens      312 

The  Vitreous  Body 314 

The  Middle  and  Outer  Tunics  of  the  Eye 315 

The  Eyelids  and  the  Lacrimal  Apparatus 318 

The  Development  of  the  Organ  of  Hearing 321 

The  Internal  Ear 321 

The  Middle  and  External  Ear 331 

The  Development  of  the  Nose 334 


CHAPTEE  XVIL 

The  Development  of  the  Muscular  System 339 

The  Striated  or  Voluntary  Muscles 339 

The  Muscles  of  the  Trunk  Proper 339 

The  Metamorphosis  of  the  Muscle-plate 342 

The  Branchial  Muscles 344 

The  Muscles  of  the  Extremities 345 

The  Involuntary  or  Unstriated  Muscular  Tissue  ....  346 


CONTENTS.  1 5 


CHAPTER  XVIII. 


PAGE 


The  Development  of  the  Skeleton  and  of  the  Limbs    ....  347 

The  Axial  Skeleton 348 

The  Development  of  the  Trunk 348 

The  Stage  of  the  Chorda 348 

The  Membranous  Stage 349 

The  Cartilaginous  Stage 35j. 

The  Osseous  Stage 354 

The  Development  of  the  Eibs  and  Sternum 356 

The  Development  of  the  Head  Skeleton 358 

The  Membranous  Cranium 359 

The  Cartilaginous  Cranium 360 

The  OsseouR  Stage 363 

The  Appendicular  Skeleton 376 

The  Pectoral  and  Pelvic  Girdles 377 

The  Bones  of  the  Extremities      378 

The  Development  of  the  Limbs 380 

The  Position  or  the  Limbs S81 

Tabulated  Chronology  of  Development 383 

Index  .   .' 391 


CHAPTER  I. 

THE  MALE  AND  FEMALE  SEXUAL  ELEMENTS; 
MATURATION  ;  OVULATION  ;  MENSTRUATION  ; 
FERTILIZATION: 

Embryology  is  that  department  of  biology  which  treats 
of  the  generation  and  development  of  organisms.  It  may 
refer  to  the  development  of  the  race  or  stock — Phylogeny — or 
to  that  of  the  individual — Ontogeny ;  again,  it  may  treat  of 
animal  or  of  vegetable  development. 

Since  no  observations  have  been  made  upon  embryos 
of  an  age  less  than  four  or  five  days,  and  but  few,  indeed, 
upon  those  younger  than  sixteen  or  eighteen  days,  we 
cannot  be  said  to  possess  definite  knowledge  of  the  very 
earliest  processes  of  development  in  man.  There  is,  how- 
ever, sufficient  analogy  between  the  known  facts  of  human 
development  and  those  of  corresponding  stages  in  allied 
groups  of  animals,  as  well  as  between  the  various  groups  of 
animals  themselves,  to  establish  certain  broad  general  princi- 
ples of  agreement  in  essential  features.  In  tracing  the  his- 
tory of  human  development,  therefore,  frequent  recourse 
must  be  had  to  the  development  of  animals,  since  in  this 
way  only  is  it  possible  at  present  to  fill  up  the  gaps  in  our 
knowledge  of  human  embryology. 

That  a  new  individual  may  be  called  into  existence,  the 
union  of  the  male  element,  or  spermatozoon,  with  the  female 
element,  or  ovum,  is  necessary.  Such  union  is  variously 
called  fertilization,  fecundation,  and  impregnation. 

Prior  to  the  beginning  of  the  present  century,  little  or 
nothing  was  definitely  known  concerning  reproduction  and 
development.  The  opinions  of  the  biologists  of  early  times 
found  expression  in  a  theory  which  was  then  called  the  theory 
of  unfolding  or  of  evolution,  but  which  more  recently  has 
2  17 


18  TEXT-BOOK  OF  EMBRYOLOGY. 

been  designated  the  preformation  theory^.  According  to  this 
doctrine,  the  egg  or  germ  contained  all  the  parts  of  the  adult 
organism  in  an  exceedingly  minute  condition,  and  develop- 
ment consisted  in  the  simple  growth  or  unfolding  of  already 
formed  parts.  As  the  theory  of  unfolding  implied  the  pre- 
formation not  only  of  the  immediate  but  of  all  subsequent 
offspring,  its  votaries  were  able  to  compute  that  the  ovary 
of  Eve  contained  200,000  millions  of  human  germs. 

With  the  discovery  of  the  spermatozoon  in  1677  by  Hamm, 
a  pupil  of  Leuwenhoeck,  a  controversy  arose  as  to  whether 
it  was  the  spermatic  filament  or  the  ovum  that  contained  the 
germ.  Those  who  maintained  the  former  view  were  known 
as  animalculists  ;  those  who  held  the  latter,  as  ovists.  Accord- 
ing to  the  opinions  of  the  animalculists,  the  spermatozoon 
was  the  complete  organism  in  miniature,  and  it  required  for 
its  growth  the  soil  or  environment  which  the  ovum  alone 
could  furnish. 

The  enunciation  by  Wolff,  in  1759,  of  his  doctrine  of  epigene- 
sis  completely  overturned  the  preformation  theory.  Wolff 
maintained  that  the  germ  was  unorganized  matter,  and  that  the 
union  of  male  and  female  material  was  essential  to  reproduc- 
tion. While  Wolff's  theory  was  in  the  main  correct,  it  re- 
mained for  later  investigators  to  show  that  the  ovum  did  not 
consist  of  unorganized  matter,  as  he  thought,  but  that  it  pos- 
sessed definite  structural  characteristics.  Thus,  the  germinal 
vesicle  of  the  hen's  egg  was  discovered  in  1825  by  Purkinje, 
and  the  germinal  spot  in  1826  by  Wagner.  Soon  after  the 
enunciation  of  the  cell-doctrine  by  Schleiden  and  Schwann, 
it  was  seen  that  the  ovum  was  in  reality  a  typical  cell,  pos- 
sessing all  the  parts  of  such  a  structure. 

It  was  not,  however,  nntil  about  the  year  1840  that  it  was 
shown,  by  K5lliker,  Reichert,  and  others,  that  the  spermatozoa 
are  the  active  agents  in  fecundation.  Previously  it  had  been 
held,  since  the  refutation  of  the  preformation  theory,  that  the 
seminal  fluid  performed  this  function,  and  that  the  spermato- 
zoa were  parasitic  organisms. 

The  length  of  time  necessary  for  the  development  of  the 
new  individual  varies  according  to  the  species ;    in  man  it 


MALE  AND  FEMALE  SEXUAL  ELEMENTS. 


19 


occupies  nine  calendar  months  or  about  ten  lunar  months — 
that  is,  from  273  to  280  days.  The  period  of  human  gesta- 
tion is  arbitrarily  divided  by  His  into  three  stages:  (1)  The 


Chromatin  part  of  para- 
nucleus. _ 

Nucleolus  in  division.       l/f  ^/-^g.^   (    ,  V,  rO^^ 


Achromatin  part  of  paranucleus. 


Fig.  1.— 1  to  8,  Various  stages  of  the  development  of  the  spermatozoon  of  the 
mouse ;  9,  the  spermatozoon  of  the  mouse  (after  F.  Hermann) ;  10  and  11,  spermato- 
zoa of  the  dog;  10,  as  seen  from  the  side  ;  11,  as  seen  from  the  broader  surface  (after 
Bonnet). 

Stage  of  tlie  ovum,  comprising  the  first  two  weeks  of  develop- 
ment ;  (2)  The  stage  of  the  embryo,  extending  from  the  end 
of  the  second  week  to  the  fifth  week,  during  which  time  the 
germ  begins  to  assume  definite  form ;  and  (3)  The  stage  of 


20 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  fetus,  which  includes  the  remainder  of  the  term  of  intra- 
uterine existence. 

It  may  be  pointed  out  that  the  term  ovuvriy  as  employed  in 
embryology,  has  three  different  significations :  it  designates 
the  female  sexual  cell  prior  to  its  impregnation  ;  it  is  used  in 
the  sense  noted  above  to  designate  the  fertilized  egg ;  and  it 
is  somewhat  loosely  applied  to  the  product  of  conception 
during  various  stages  of  development. 


A 


THE  SPERMATOZOON. 

It  is  noteworthy  that  both  spermatozoa  and  ova — that  is, 
both  sexual  cells — are  products  of  metamorphoses  taking 
place  in  epithelial  structures,  the  former  being  derived  from 
the  spermatogenic  cells  found  in  the  seminiferous  tubules  of 
the  testicle,  while  the  latter  come  from 
the  germinal  epithelium  of  the  ovary. 

The  form  of  the  seminal  filament  va- 
ries greatly  in  different  species  (Fig.  1). 
The  human  spermatozoon  (Fig.  2)  is  an 
elongated  body,  about  0.05  mm.  (5^ 
inch)  in  length,  consisting  of  three  parts, 
a  head,  a  middle  piece,  and  a  tail  or 
flagellum. 

The  head  is  much  thickened  as  com- 
pared with  the  other  segments,  appear- 
ing egg-shaped  as  seen  upon  its  broader 
surface,  the  smaller  extremity  being 
connected  with  the  middle  piece ;  seen 
in  profile,  it  is  convex  on  one  side  and 
concave  on  the  other.  The  middle 
piece  is  somewhat  longer  and  much 
thinner  than  the  head,  while  the  tail 
is  a  slender  filament  slightly  more  than 
four-fifths  of  the  entire  length  of  the 
spermatozoon.  Lying  in  the  center 
of  the  spermatozoon,  and  extending 
throughout  its  entire  length,  is  the  slender  axial  fiber,  which 
is  prolonged  slightly  beyond  the  tail. 


sper- 


FiG.   2.— Human 
matozoa    (after   Retzius) : 

A,  spermatozoon  seen  en 
face;  A,  head;  m,  middle- 
piece  ;  t,  tail ;  e,  end-piece ; 

B,  C,  seen  from  the  side. 


THE  SPERMATOZOON. 


21 


The  power  of  locomotion  which  the  spermatozoon  exhibits 
is  conferred  by  the  vibratile  movement  of  its  tail,  accompanied 
by  a  rotation  about  its  long  axis  through  an  arc  of  90  degrees. 
The  rate  of  progression  is  about  0.05  or  0.06  mm.,  or  its  own 
length,  per  second. 

Spermatozoa  possess  remarkable  vitality,  remaining  active 
in  the  male  genital  tract  for  several  days  after  death.  In  the 
genital  passages  of  the  female,  they  may  retain  their  activity 
for  several  weeks,  and  when  mounted  and  protected  from 
evaporation  they  have  been  known  to  show  vibratile  motion 
after  the  lapse  of  nine  days  (Piersol).  Weak  alkaline  solu- 
tions render  them  more  active,  while  acids,  even  quite  dilute, 
destroy  them.  The  spermatozoa  of  the  bat,  being  deposited 
in  the  female  genital  passages  in  the  autumn,  retain  their 
power  of  fecundating  ova  until  the  following  spring. 

Spermatogenesis. — The  details  of  spermatozoon-formation, 
or  spermatogenesis,  vary  in  different  animals.  A  cross  sec- 
tion of  a  seminiferous   tubule    (Fig.  3)  shows  a  layer  of 


Fig.  3.— Section  of  testicle  of  musk-rat ;  seminiferous  tubule  seen  in  cross  section : 
a,  wall  of  tubule ;  6,  parietal  cells ;  c,  mother-cells ;  d,  spermatoblasts. 

cuboidal  cells  called  parietal  cells,  lying  in  contact  with 
the  basement  membrane  of  the  tubule  wall.  This  layer  con- 
sists of  the  so-called  Sertoli's  columns,  or  sustentacular  cells, 
and  of  the  spermatogenic  cells.  The  sustentacular  cells  are 
merely  supporting ;  the  spermatogenic  cells  give  rise  to  the 
spermatozoa.     The  immediate  oiFspring  of  the  spermatogenic 


22  TEXT-BOOK  OF  EMBRYOLOGY. 

cells  are  the  mother-cells,  a  group  lying  in  contact  with  the 
spermatogenic  cells,  on  the  side  toward  the  lumen  of  the 
tubule.  The  multiplication  of  the  mother-cells  results  in 
the  production  of  the  smaller  daughter-cells,  whose  nuclei 
are  the  spermatoblasts,  or  spermatids  of  some  authors.  The 
round  spermatoblast  soon  becomes  oval,  and  later  pear-shaped, 
and  its  chromatin  accumulates  in  the  outer  half  of  the  nu- 
cleus, giving  rise  to  a  dark,  deeply-staining  zone  in  this  posi- 
tion, and  a  clear  zone  on  the  side  toward  the  lumen  of  the 
tubule.  At  the  same  time  the  nucleus  or  spermatoblast 
escapes  from  the  daughter-cell.  A  little  later  the  sperma- 
toblast shows  three  zones — an  outer  clear  band,  a  middle 
dark  area,  in  which  the  chromatin  has  become  localized,  and 
an  inner  clear  region.  A  slender  filament  or  spine  of  chro- 
matin, the  rudiment  of  the  tail,  now  grows  from  the  middle 
dark  zone  into  and  through  the  inner  clear  area  (that  is, 
toward  the  lumen  of  the  tubule),  projecting  slightly  beyond 
the  limits  of  the  nuclear  membrane.  With  the  disappear- 
ance of  the  nuclear  membrane  and  the  progress  of  further 
changes — which  changes  consist  in  the  formation  of  the  head 
from  the  outer  clear  band,  of  the  tail  and  middle  piece  from 
the  middle  dark  zone,  and  of  the  delicate  sheath  for  the  tail 
and  the  middle  piece  from  the  inner  clear  band — the  develop- 
ment of  the  spermatozoon  is  practically  completed. 

The  spermatozoon  has  been  regarded  as  a  metamorphosed 
cell,  the  head  representing  the  nucleus,  and  the  cilium 
or  tail  the  protoplasm.  This  view,  however,  is  contro- 
verted by  certain  authorities  (Kolliker,  Biondi,  Niessing, 
Piersol),  and  the  weight  of  the  evidence  is  in  favor  of  regard- 
ing the  entire  spermatozoon  as  having  been  developed  from  a 
cell-nucleus. 

THE  OVUM. 

The  female  sexual  cell  or  ovum  is  remarkable  among  animal 
cells  for  its  size,  it  being  a  rule,  to  which  there  are  no  known 
exceptions,  that  it  is  much  larger  than  any  other  cell  in  the 
body  of  the  parent.  The  human  ovum  measures,  in  the  mature 
state,  0.2  mm.  in  diameter. 

In  structure,  the  ovum  presents  the  parts  of  a  typical  cell ; 


THE  OVUM. 


23 


namely,  a  cell- wall,  here  called  the  vitelline  membrane,  the 
cell-contents,  or  vitellus  or  yolk,  a  nucleus  or  germinal  vesi- 
cle, and  a  nucleolus  or  germinal  spot. 

Surrounding  the  ovum  is  a  somewhat  loosely-fitting  trans- 
parent, elastic  envelope,  the  zona  pellucida,  and  outside  of 
this  is  the  corona  radiata.     These  two  layers  are  often  re- 


\ 


,y 


Fig.  4.— Egg  from  a  rabbit's  follicle  which  was  0.2  mm.  (yi^inch)  in  diameter 
(after  Waldeyer).  It  is  surrounded  by  the  zona  pellucida  (s.  p.),  on  which  there 
rest  at  one  place  follicular  cells  (/.  z.).  The  yolk  contains  deutoplasmic  granules 
(d.).  In  the  germinative  vesicle  (k.  b.)  the  nuclear  network  (fc.  n.)  is  especially 
marked,  and  contains  a  large  germinative  dot  (k.f.). 

ferred  to  as  the  egg-envelopes  ;  and  since  they  are  contributed 
by  the  discus  proligerus  of  the  Graafian  follicle,  it  must  be 
remembered  that  they  are  not,  properly  speaking,  a  part  of 
the  ovum.  Between  the  zona  pellucida  and  the  ovum  is  the 
small  perivitelline  space.  The  radial  striation  of  the  zona  is 
generally  regarded  as  due  to  the  presence  of  minute  canals 
opening  into  this  space.  The  canals  are  thought  by  some  to 
facilitate  the  ingress  of  spermatozoa,  thus  corresponding  in 
function  to  the  micropyle,  a  small  aperture  found  in  the  less 
easily  penetrable  egg-envelopes  of  many  invertebrates  and 
of  some  fishes. 


24 


TEXT-BOOK  OF  EMBRYOLOGY. 


The  vitelline  memlDrane  does  not  call  for  extended  descrip- 
tion. It  may  be  regarded  as  a  slightly  specialized  condensa- 
tion of  the  peripheral  part  of  the  cell-contents. 

The  vitellus,  or  cell-contents,  here,  as  in  other  cells,  is 
essentially  protoplasm,  to  which  is  added  material  called  deu- 
toplasm,  designed  for  the  nutrition  of  the  ovum  at  the  begin- 
ning of  development.  The  protoplasm  is  also  called  the 
formative  yolk  and  the  egg-plasm,  while  the  deutoplasm  is 
known  as  the  nutritive  yolk.  In  the  human  ovum  these  ele- 
ments are  more  or  less  uniformly  distributed  ;  there  is,  how- 
ever, a  differentiation  into  an  inner,  slightly  less  clear  region, 
containing  more  yolk-granules  (deutoplasm),  and  a  peripheral, 
clearer  zone.  The  characteristic  transparency  of  the  human 
egg-cell  is  due  to  the  fact  that  the  deutoplasmic  particles 
found  in  it  are  not  cloudy  as  in  the  ova  of  other  mammals. 
The  following  classification  of  ova  by  Balfour  is  based  upon 
the  arrangement  of  these  constituents  : 

1.  Alecithal  ova  are  those  in  which  the  protoplasm  and 


Fig.  5.— Diagram  of  an  egg  with  the 
nutritive  yolk  in  a  polar  position.  The 
formative  yolk  constitutes  at  the  animal 
pole  {A.  P.),  a  germ-disk  {K.sch.)  in  which 
the  germinative  vesicle  (fc.  6.)  is  en- 
closed. The  nutritive  yolk  {n.d.)  fills 
the  rest  of  the  egg  up  to  the  vegetative 
pole  (V.P.)  (Hertwig). 


Fig.  6.— Diagram  of  an  egg  with  the 
nutritive  yolk  in  the  center.  The 
germinative  vesicle  (fc.  6.)  occupies  the 
middle  of  the  nutritive  yolk  {n.  d.), 
which  is  enveloped  in  a  mantle  of 
formative  yolk  (6.  d.)  (Hertwig). 


deutoplasm  are  uniformly  distributed,  as  in  the  ova  of  Mam- 
malia, including  man),  and  of  amphioxus  (Fig.  4). 

2.  Telolecithal  ova  are  those  in  which  the  relatively  abun- 


THE  OVUM.  25 

dant  deutoplasm  is  accumulated  at  one  side  of  the  ovum, 
called  the  vegetative  pole,  while  the  protoplasm  appears  as  a 
flat  germ-disk  at  the  animal  pole  on  the  opposite  side.  Here 
belong  the  eggs  of  birds,  reptiles,  and  bony  fishes  (see  Fig.  5). 

3.  Centrolecithal  ova  are  those  in  which  the  deutoplasm  is 
central,  the  protoplasm  completely  surrounding  it,  as  in  the 
eggs  of  arthropods  (Fig.  6). 

Ova  are  classified  also  according  to  their  method  of  seg- 
mentation.    This  will  be  described  later. 

The  germinal  vesicle  or  nucleus  is  the  most  important  part 
of  the  cell,  since,  as  will  be  seen  hereafter,  it  is  essentially 
by  the  conjugation,  or  more  accurately  by  the  fusion,  of  the 
nuclei  of  the  male  and  female  parent-cells  that  generation  is 
efiFected.  As  a  rule,  there  is  but  one  nucleus,  though  there 
may  be  two.  Its  position  is  usually — if  not  universally — 
eccentric,  this  being  more  marked  where  there  is  a  distinct 
differentiation  into  animal  and  vegetative  poles,  in  which  case 
it  is  found  always  near  the  animal  pole.  It  is  nearly  spheri- 
cal in  shape,  and  like  the  nucleus  of  any  other  typical  cell, 
it  is  composed  of  a  network  of  nuclear  fibrils  or  chromatin 
substance,  and  nuclear  juice  or  achromatin,  the  former  con- 
taining the  latter  within  its  meshes.  Surrounding  the  nucleus 
is  the  well-marked  nuclear  membrane,  while  within  it  is  the 
nucleolus  or  germinal  spot.  The  latter  may  be  single  or 
multiple,  according  to  the  species,  though  the  number  is  fairly 
constant  for  each  species.  Nagel  ascribes  ameboid  move- 
ment to  the  germinal  spot. 

Polarity. — The  polarity  of  the  eg^  has  been  incidentally 
referred  to.  Apparently  it  owes  its  existence  to  the  eccentric 
position  of  the  nucleus,  the  animal  pole  being  that  point  on 
the  surface  to  which  the  nucleus  is  nearest.  Polarity  bears  a 
significant  relation  to  the  specific  gravity  of  the  ovum,  since 
the  nucleus  reaches  the  surface  of  the  latter  at  the  animal 
pole  and  there  extrudes  the  polar  globules ;  and  it  is  also 
related  to  the  segmentation  of  the  fertilized  egg. 

The  Hen's  Bgg. — As  the  hen's  egg  is  so  largely  utilized 
for  the  study  of  development,  it  will  be  profitable  to  consider 
briefly  its  structure.     The  ovum  or  egg-cell  is  represented  by 


26 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  yolk  or  yellow  of  the  egg,  the  albumen  or  white,  as  well 
as  the  shell  and  shell-membrane,  being  egg-envelopes  con- 
tributed by  the  oviduct.  As  in  other  ova,  the  egg  proper  is 
a  single  cell,  having  a  vitelline  membrane  and  a  germinal 
vesicle.  The  enormous  size  of  the  cell  is  due  to  the  large 
quantity  of  nutritive  material  or  deutoplasm  present,  this 
contributing  by  far  the  greater  part  of  the  bulk,  while  the 
much  smaller  formative  yolk  or  protoplasm,  containing  the 
germinal  vesicle,  is  so  eccentrically  placed  that  it  seems  to 
float  upon  the  surface  of  the  deutoplasm.  The  little  whitish 
spot  on  the  surface  of  the  yolk,  known  as  the  cicatricula  or 
germinative  disk,  consists  of  the  germinal  vesicle  with  the 
surrounding  formative  yolk.  It  is  in  the  germinative  disk 
alone  that  segmentation  takes  place,  and  it  is  for  this  reason 
that  eggs  of  this  class  are  designated  meroblastic,  or  partially- 
dividing  eggs. 

The  deutoplasm  is  made  up  of  white  and  of  yellow  yolk 


Fig.  7.— Diagrammatic  longitudinal  section  of  an  unincubated  hen's  egg  (after 
Allen  Thomson).  (Somewhat  altered):  b.l,  germ-disk;  w.y,  white  yolk,  which 
consists  of  a  central  flask-shaped  mass,  and  a  number  of  concentric  layers  sur- 
rounding the  yellow  yolk  iy.y.);  v.t,  vitelline  membrane;  z,  a  somewhat  fluid 
albuminous  layer  which  immediately  envelopes  the  yolk ;  w,  albumen,  composed 
of  alternating  layers  of  more  and  less  fluid  portions;  ch.l,  chalazse ;  a.ch,  air- 
chamber  at  the  blunt  end  of  the  egg— simply  a  space  between  the  two  layers  of 
the  shell-membrane;  i.s.m,  inner,  s.m,  outer  layer  of  the  shell-membrane;  s, 
sheU. 


(Fig.  7).     The  former  consists  of  a  thin  layer  spread  over 
the  surface  of  the  latter ;  of  a  small  mass,  known  as  Pander's 


THE   OVUM.  27 

nucleus,  situated  under  the  germinative  disk;  of  a  larger 
mass,  the  latebra,  more  deeply  placed ;  and  of  several  con- 
centric layers  separated  from  each  other  by  the  yellow  yolk. 

Such  is  the  egg  as  it  leaves  the  hen's  ovary.  In  the  begin- 
ning of  the  oviduct  it  is  fertilized  by  the  spermatozoa  already 
there.  After  fertilization  it  passes  into  the  longitudinally 
furrowed  second  part  of  the  tube,  where  it  receives  a  copious 
coating  of  albuminous  material,  the  white  of  the  &gg ;  thence 
it  goes  into  the  villous  third  part  of  the  oviduct,  where  it 
acquires  a  calcareous  coating,  the  shell;  finally,  passing 
through  the  fourth  part  of  the  canal,  it  is  "  laid." 

The  layer  of  albumen  immediately  surrounding  the  yolk 
is  relatively  dense ;  it  is  prolonged  to  either  extremity  of  the 
egg,  somewhat  spirally  twisted,  as  the  chalazse.  Enclosing 
the  albumen  is  the  thin  tough  shell-membrane.  This  con- 
sists of  two  layers,  which  separate  at  the  blunt  pole  of  the 
egg  soon  after  it  is  laid,  giving  rise  to  the  air-chamber.  The 
shell,  composed  largely  of  lime  salts,  is  very  porous  and  thus 
readily  permits  of  the  necessary  gas-interchange  between  the 
contents  of  the  egg  and  the  external  air  during  incubation. 

Ova  do  not  possess  the  remarkable  vitality  which  is  char- 
acteristic of  spermatozoa.  An  unimpregnated  ovum  per- 
ishes in  from  seven  to  nine  days. 

Oogenesis. — The  formation  of  ova  takes  place  throughout 
the  greater  part  of  fetal  life  and  continues  for  a  short  time 
(two  years,  according  to  Waldeyer,  Bischoif,  and  others) 
after  birth.  Their  number  is  estimated  to  be  about  seventy 
thousand. 

The  ovum,  the  direct  derivative  of  the  germinal  epithe- 
lium covering  the  free  surface  of  the  ovary,  is  situated  in  the 
cortical  part  of  the  latter  organ,  being  enclosed  in  the 
Graafian  follicle.  As  a  rule,  each  Graafian  follicle  or  ovi- 
sac contains  but  one  ovum,  though  sometimes  two,  and  more 
rarely  three  are  present. 

The  Grraafian  follicle,  in  its  mature  condition,  is  a  vesicle 
from  4  to  8  mm.  in  diameter,  which  is  surrounded  by  a 
sheath,  the  theca  foUiculi  or  tunica  vasculosa,  consisting  of  a 
condensation  of  the  ovarian  stroma.     The  outer,  more  fibrous 


28 


TEXT-BOOK  OF  EMBRYOLOGY. 


Fig.  8. — Section  of  human  ovary,  including  cortex :  a,  germinal  epithelium  of 
free  surface ;  6,  tunica  albuginea ;  c,  peripheral  stroma  containing  immature 
Graafian  follicles  (d) ;  e,  well-advanced  follicle  from  whose  wall  membrana  granu- 
losa has  partially  separated  ;  /,  cavity  of  liquor  folliculi;  g,  ovum  surrounded  by 
cell-mass  constituting  discus  proligerus  (Piersol). 

zone  of  the  theca,  containing  large  blood-vessels,  is  distin- 
guished as  the  tunica  fibrosa ;  the  inner  more  cellular  layer, 
rich  in  small  vessels  and  capillaries,  as  the  tunica  propria. 


Fig.  9.— Section  of  well-developed  Graafian  follicle  from  human  embryo  (Von 
Herff ) ;  the  enclosed  ovum  contains  two  nuclei. 


TEE  OVUM.  29 

The  fibrous  wall  of  the  follicle  is  lined  by  the  membrana 
granulosa,  which  consists  of  many  layers  of  epithelial  cells ; 
these,  at  the  point  of  contact  with  the  ovum,  project  in  such 
a  manner  as  to  surround  it  completely,  the  cellular  envelope 
thus  formed  constituting  the  discus  proligerus.  The  inner 
cells  of  the  discus  are  arranged  in  two  layers,  the  individual 
elements  having  their  long  axes  radially  directed.  From  the 
appearance  of  radial  striation,  conferred  partly  by  this  cir- 
cumstance, the  inner  zone  has  been  called  the  zona  radiata 
or  zona  pellucida,  and  the  outer  the  corona  radiata.  The 
cavity  of  the  Graafian  follicle  is  filled  with  fluid,  the  liquor 
foUiculi. 

The  stigma,  or  Mlum  folliculi,  a  yellowish-white  spot  devoid 
of  blood-vessels  on  the  free  surface  of  the  Graafian  follicle, 
indicates  the  point  at  which  rupture  will  take  place.  After 
this  event,  which  occurs  when  the  ovum  is  "  ripe,"  the  latter 
passes  into  the  Fallopian  tube. 

The  ultimate  origin  of  the  egg  is  to  be  sought  in  that  im- 
portant group  of  cells  on  the  surface  of  the  ovary  to  which 
Waldeyer  gave  the  name  germinal  epithelium.  This  first 
appears  at  about  the  fifth  week  of  intra-uterine  life,  as  a 
localized  thickening  of  the  cells  of  the  structure  that  subse- 
quently becomes  the  peritoneum.  The  thickened  areas  com- 
prise two  longitudinal  elevations  on  the  dorsal  side  of  the 
future  abdominal  cavity,  one  on  each  side  of  the  median 
plane  of  the  body ;  these  are  the  genital  ridges.  Owing  to 
the  development  of  connective  tissue  beneath  the  epithelium, 
the  ridges  increase  in  thickness,  and,  with  the  progress  of 
other  changes,  finally  become,  in  the  female,  the  ovaries. 
At  about  the  sixth  or  seventh  week — the  germinal  epithelium 
now  consisting  of  several  layers  of  cells  instead  of  being  a 
single  stratum  thick,  as  at  first — cord-like  processes,  the 
sexual  cords,  or  primary  egg-tubes,  or  egg-columns,  grow  from 
the  surface  into  the  underlying  connective  tissue,  carrying  with 
them  certain  of  the  surface-cells  (see  Fig.  112).  Conspicuous 
among  these  are  the  large  sexual  cells,  or  primitive  ova ;  while 
smaller  cells,  likewise  from  the  germinal  epithelium,  are 
also  present.     The  sexual  cords  become  divided  into  groups 


30  TEXT-BOOK  OF  EMBRYOLOGY. 

of  cells,  each  group  containing  one  or  more  primitive  ova 
and  many  of  the  smaller  cells.  Gradually,  the  small  cells 
of  the  group  surround  the  primitive  ovum,  at  first  as  a  single 
layer  of  flattened  cells,  which  are  succeeded  by  several  layers 
of  polygonal  cells.  From  these  enveloping  cells  come  the 
membrana  granulosa  and  the  theca  of  the  Graafian  follicle. 

The  primitive  ova  become  fully  formed  eggs  upon  the 
assumption  by  their  rather  ill-defined  nuclei  of  the  charac- 
teristic shape  and  structure  of  typical  nuclei,  coincidentally 
with  the  occurrence  of  other  changes  of  secondary  importance 
in  other  parts  of  the  cell. 

The  youngest  ova  are  found  nearest  the  surface  of  the 
ovary,  the  eggs  as  they  develop  advancing  toward,  but  never 
entering,  the  medulla  of  the  organ.  Finally,  in  the  fully- 
developed  condition  of  the  ovum  and  the  follicle,  the  size  of 
the  latter  is  such  that  its  diameter  equals  or  exceeds  the 
thickness  of  the  ovarian  cortex,  its  position  being  usually 
indicated  by  a  small  prominence  on  the  surface  of  the  ovary. 

MATURATION   OF  THE  OVUM. 

By  maturation  or  ripening  is  meant  that  series  of  changes 
by  which  the  ovum  is  prepared  for  fertilization  and  without 
which  the  latter  process  is  impossible.  In  nearly  all  mam- 
mals, including  man,  it  occurs  while  the  ovum  is  still  in  the 
Graafian  follicle;  in  some  other  groups  it  takes  place  after 
the  Qgg  has  reached  the  oviduct. 

Briefly,  maturation  may  be  said  to  consist  in  the  extrusion 
from  the  cell  of  a  part  of  its  nucleus  and  of  a  small  pan  of 
its  protoplasm.  The  nucleus  undergoes  changes  practically 
identical  with  those  of  ordinary  cell-division.  First,  the 
nuclear  membrane  disappears,  the  nucleolus  disintegrates, 
the  nuclear  juice  becomes  mingled  with  the  surrounding  pro- 
toplasm, and  the  nucleus  moves  toward  the  periphery  of  the 
egg  (Fig.  10).  There  is  now  formed  a  nuclear  spindle  from 
the  achromatin  substance  of  the  nucleus.  The  long  axis  of 
the  spindle  lies  parallel  with  one  of  the  radii,  and  its  direc- 
tion is  determined  by  the  position  of  the  pole-corpuscles. 
Each  pole-corpuscle  is  surrounded  by  a  radiation,  the  attrac- 


MATURATION  OF  THE  OVUM. 


31 


tion-sphere  or  polar  striation.     These  bodies  exercise  a  con- 
trolling influence  upon  the  nuclear  spindle,  so  that  it  assumes 


':-d 


—b 


'>5Sf, 


6— 


o 


Fit.  10  — I'ortioiis  of  the  ova  of  Asterias  glacialis,  showing  changes  affecting  the 
germinal  \e&icle  at  the  beginning  of  maturation  (Hertwig):  a,  germinal  vesicle; 
b,  germinal  spot,  composed  of  nuclein  and  paranuclein  (c) ;  d,  nuclear  spindle  in 
process  of  formation. 

such  a  position  that  each  of  its  apices  points  toward  a  pole- 
corpuscle. 

The  outer  extremity  of  the  nuclear  spindle,  being  made  to 
protrude  by  the  continued  onward  movement  of  the  nucleus, 
becomes  detached  (Fig.  11);  this  separated  piece,  with  the 


Fig.  11.— Formation  of  the  polar  bodies  in  the  ova  of  Asterias  glacialis  (Hert- 
wig): ps,  polar  spindle;  pV,  first  polar  body ;  pb",  second  polar  body;  n,  nucleus 
returning  to  condition  of  rest. 

small  surrounding  constricted-off  mass  of  protoplasm,  con- 
stitutes the  first  polar  body.  From  the  remnant  of  the  first 
nuclear  spindle,  a  second  one  is  formed,  which  in  the  same 
manner  extrudes  the  second  polar  body.  What  remains  of 
the  nucleus  now  moves  toward  the  center  of  the  cell  and 


32 


TEXT-BOOK  OF  EMBRYOLOGY. 


temporarily  disappears,  soon  to  reappear  as  the  female  pro- 
nucleus. The  position  of  the  female  pronucleus  is  nearly  or 
absolutely  central.  The  protoplasm  surrounding  it  is  radially 
striated  (Fig.  12).     The  egg  is  now  ready  for  fertilization. 


Fig.  12.— a,  mature  ovum  of  echinus ;  n,  female  pronucleus ;  B,  immature  ovarian 
ovum  of  echinus  (Hertwig). 

The  fate  of  the  polar  bodies  is  undetermined.  For  some 
time  after  their  extrusion,  and  pending  their  final  disappear- 
ance, they  are  to  be  seen  lying  in  the  perivitelline  space. 
The  formation  of  polar  globules  is  probably  almost  universal 
throughout  the  animal  world,  although  in  reptiles  and  birds 
and  in  some  fishes  and  amphibians  they  have  not  as  yet  been 
demonstrated.  It  is  of  interest  to  note  that  in  some  par- 
thenogenetic  eggs — that  is,  eggs  capable  of  developing  into  a 
new  individual  without  contact  with  the  male  element,  as  for 
example  the  summer  eggs  of  plant  lice  and  of  some  other 
arthropods — only  one  polar  globule  is  said  to  be  formed. 

One  of  the  theories  advanced  to  explain  the  phenomenon 

of  maturation  is  that  of  Minot.     This  theory  assumes  that 

the  ovum  is  at  first  hermaphroditic — that  is,  that  it  contains 

both  male  and  female  elements,,  and  that  the  extrusion  of  the 

polar  bodies  is  a  casting-off  of  the  male  element.     Hertwig 

and  others  believe,  however,  that  the  formation  of  the  polar 

bodies  is  an  abortive  cell-division,  the  bodies  being  aborted 

cells. 

OVULATION. 

Extrusion  of  the  ovum  from  the  Graafian  follicle,  or  ovu- 
lation, occurs  upon  the  completion  of  the  process  of  matura- 


OVULATION,  33 

tion.  As  the  time  for  this  event  approaches,  the  wall  of  the 
follicle  at  the  site  of  the  stigma  becomes  much  thinned  and 
finally  ruptures,  and  the  ovum  passes  into  the  Fallopian 
tube  (Fig.  13).      If,  instead  of  passing  into  the  tube,  the 


Fig.  13.— Ovary  with  mature  Graafian  follicle  about  ready  to  burst  (Ribemont- 

Dessaignes). 

ovum  maintains  its  connection  with  the  ovary  and  is  fertil- 
ized there,  it  may  undergo  partial  development  in  situ  ;  such 
a  condition  constitutes  one  variety  of  extra-uterine  pregnancy 
or  ectopic  gestation. 

Ova  are  extruded  from  the  ovary,  one  or  more  at  a  time, 
at  regular,  generally  monthly,  intervals,  from  puberty  to  the 
climacteric. 

After  the  escape  of  the  ovum,  hemorrhage  into  the  empty 
follicle  occurs,  the  resulting  clot  being  the  corpus  hemorrhagi- 
cum.  According  to  Leopold,  if  rupture  occurs  during  the 
intermenstrual  period  instead  of  at  the  time  of  menstruation,, 
hemorrhage  will  be  small  or  entirely  wanting,  the  resultiug^ 
corpus  luteum  being  called  then  atypical,  to  distinguish  it 
from  the  typical  body  formed  in  the  ordinary  manner. 

The  blood-clot  is  soon  permeated  by  cells  originating  ia 
the  wall  of  the  follicle,  some  of  which  are  fusiform  con- 
nective-tissue cells,  while  others  are  large  cells  containing 
the  yellow  pigment,  lutein.  Meanwhile,  the  follicular  wall! 
thickens  and  becomes  plicated.  Later,  upon  the  replacement 
of  the  mass  of  clot  and  cells  by  fibrous  tissue  and  the  devel- 
opment of  capillaries  within  it,  the  body  assumes  a  yellowish 
cicatricial  appearance  and  is  known  as  the  corpus  luteum. 
(Fig.  14).     The  color  of  the  corpus  varies  considerably  in 


34  TEXT-BOOK  OF  EMBRYOLOGY. 

different  species  of  animals,  the  yellow  color  being  character- 
istic for  the  human  subject. 

If  the  ovum  is  not  fertilized,  the  corpus  luteum  attains  its 
maximum  development  in  less  than  a  week  and  begins  to 
shrink  at  about  the  twelfth  day,  becoming  completely  ab- 
sorbed in  a  few  weeks.  If  fertilization  occurs,  it  continues 
to  grow  for  two  or  three  months  and  acquires  a  size  one- 
fourth  or  one-third  that  of  the  entire  ovary;  persisting  till 


Fig.  14.— Ovaries  of  two  virgins,  showing  large  corpora  lutea,  resembling  those  of 
pregnancy  (Hirst). 

toward  the  end  of  gestation,  it  finally  shrinks  to  a  small 
white  scar,  which  may  not  totally  disappear  until  a  month  or 
more  after  labor. 

It  has  been  customary  to  designate  the  larger,  better  devel- 
oped yellow  body,  the  true  corpus  luteum,  or  the  corpus 
luteum  of  pregnancy,  in  contradistinction  to  the  so-called 
false  corpus  luteum  of  menstruation,  and  to  regard  the  pres- 
ence of  the  former  as  absolute  proof  of  previous  impregna- 
tion. This  view  is  no  longer  tenable,  since  bodies  identical 
in  appearance  with  true  corpora  lutea  have  been  found  in 
virgin  ovaries  (Hirst). 

The  relation  of  ovulation  to  the  menstrual  function  has 
been  much  discussed.  While  the  two  processes  usually  occur 
at  the  same  time,  they  are  not  to  be  regarded  as  dependent 
one  upon   the  other.     It  has  been  shown  by  Coste,  whose 


MENSTRUATION.  35 

observations  have  been  confirmed  by  Leopold,  that  as  a  rule 
Graafian  fi^llicles  burst  during  menstruation,  though  they  may 
rupture  before  or  after  this  event.  It  has  also  been  shown 
that  in  the  rabbit  sexual  intercourse  hastens  the  rupture  of 
the  follicle. 

MENSTRUATION. 

Menstruation,  or  the  catamenial  flow,  is  considered  here 
because  of  its  natural  association  with  the  function  of  ovu- 
lation. 

Menstruation  may  be  defined  as  a  periodical  discharge  of 
blood  and  disintegrated  epithelium  and  other  structural  ele- 
ments of  the  mucous  membrane  of  the  body  of  the  uterus, 
mixed  with  mucus  from  the  uterine  glands  and  the  vagina, 
occurring  normally  about  every  twenty-eight  days,  and 
associated  with  more  or  less  disturbance  of  the  entire  sexual 
system.  The  inauguration  of  the  function  marks  the  age  of 
puberty,  the  beginning  of  the  sexual  life  of  woman ;  its  ces- 
sation, known  as  the  climacteric,  or  menopause,  indicates  the 
termination  of  the  child-bearing  period. 

In  temperate  climates,  the  menses  are  established  between 
the  thirteenth  and  seventeenth  years  and  cease  between  the 
ages  of  forty  and  fifty.  In  the  tropics,  they  appear  some- 
what earlier ;  in  cold  climates,  somewhat  later.  The  function 
is  suspended  during  pregnancy  and,  usually,  during  lactation. 

The  quantity  of  the  discharge,  though  subject  to  consider- 
able variation,  is  usually  from  4  to  6  fluidounces.  The  blood 
is  venous  in  character,  and,  owing  to  admixture  of  alka- 
line mucus,  does  not  coagulate  unless  present  in  excessive 
amount. 

The  menstrual  cycle  of  twenty-eight  days  may  be  di- 
vided into  four  periods :  the  constructive  stage,  comprising 
from  five  to  seven  days ;  the  destructive  stage,  lasting  about 
five  days ;  the  stage  of  repair,  covering  a  period  of  three  or 
four  days ;  and  the  stage  of  (juiescence,  including  the  remain- 
ing twelve  to  fourteen  days. 

In  the  constructive  stage,  which  occupies  the  six  to  seven 
days  preceding  the  discharge,  the  mucous  membrane  of  the 


36  TEXT-BOOK  OF  EMBRYOLOGY. 

uterus  becomes  markedly  swollen,  the  normal  thickness  of 
from  1  to  2  millimeters  being  more  than  doubled.  The  ute- 
rine glands  become  wider  and  longer  and  also  more  branched. 
The  blood-vessels,  especially  the  capillaries  and  veins,  un- 
dergo great  increase  in  size,  and  the  connective-tissue  cells 
are  increased  in  number.  The  thickened  raucous  membrane 
resulting  from  these  alterations  is  the  decidua  menstrualis. 
The  term  "  constructive  "  is  applied  to  this  series  of  changes 
for  the  reason  that  their  apparent  purpose  is  the  preparation 
of  the  womb  for  the  reception  of  a  fertilized  ovum. 

The  destructive  stage,  corresponding  to  menstruation 
proper,  lasts  from  three  to  five  days.  It  consists  essentially 
in  the  partial  destruction  of  the  hypertrophied  mucous  mem- 
brane, the  menstrual  decidua,  accompanied  by  hemorrhage. 
The  initial  step  is  the  infiltration  of  blood  into  the  subepi- 
thelial tissue  ;  according  to  Overlach,  this  takes  place,  not  by 
rupture  of  capillaries,  but  by  diapedesis.  In  a  day  or  two 
the  superficial  layers  of  the  mucous  membrane  disintegrate 
and  are  cast  off,  those  portions  of  the  enlarged  uterine  glands 
included  within  this  stratum  sharing  the  same  fate.  By  the 
loss  of  the  epithelium  and  the  subjacent  strata,  the  blood- 
vessels are  exposed.  Subsequently  these  rupture,  giving  rise 
to  the  characteristic  hemorrhage.  Fatty  degeneration  accom- 
panies the  death  of  the  cast-off  tissue,  and  was  thought  by 
Kundrat  and  Engelman  to  be  the  direct  cause  of  the  hemor- 
rhage ;  it  is  probable,  however,  that  fatty  degeneration  is  not 
present  until  after  the  flow  of  blood  has  begun. ^ 

The  stage  of  repair,  comprising  the  three  or  four  days  fol- 
lowing the  period  of  the  discharge,  witnesses  the  return  of 
the  uterine  mucosa  to  its  usual  condition.  With  the  gradual 
subsidence  of  the  swelling,  the  superficial  layers,  which  were 
lost,  are  replaced  by  the  growth  of  new  tissue  from  the 
deeper  layers,  which  persisted.  The  formation  of  the  new 
epithelium  begins  at  the  mouths  of  the  uterine  glands. 

The  stage  of  quiescence  extends  from  the  close  of  the  pre- 

^  Marshall's  "Vertebrate  Embryology;"  Minot's  "Human  Embryol- 
ogy-" 


MENSTR  UA  TION.  3  7 

ceding  stage  to  the  end  of  the  cycle,  or,  in  other  words,  to 
the  beginning  of  the  next  constructive  stage. 

Other  j>arts  of  the  sexual  apparatus,  including  the  ovaries, 
the  Fallopian  tubes,  and  the  mammary  glands,  show  more  or 
less  sympathy  with  the  uterus  during  menstruation,  the 
changes  in  them  consisting  chiefly  in  swelling,  hyperemia, 
and  tenderness. 

The  Relation  of  Menstruation  to  Ovulation  and 
Conception. — The  function  of  menstruation  and  the  ex- 
trusion of  ova  from  the  Graafian  follicles,  though  closely 
associated,  are  not  dependent  upon  each  other.  Ovulation 
occurs  perhaps  most  commonly  during  the  time  of  the  men- 
strual discharge,  but  it  may  take  place  before  or  after  this 
event.  While  it  is  now  generally  accepted  that  the  two 
functions  are  not  mutually  interdependent  in  the  sense  that 
one  is  a  necessary  part  of  the  other,  yet,  since  the  turgescence 
incident  to  sexual  intercourse  has  been  shown  to  hasten  the 
rupture  of  the  follicles,  it  seems  reasonable  to  suppose  that 
the  ovarian  hyperemia  attendant  upon  the  menstrual  epoch 
would  exert  a  like  influence. 

Since  the  function  of  menstruation  is  normally  suspended 
during  pregnancy,  the  relation  between  menstruation  and 
ovulation,  and  of  these  to  conception,  are  of  practical  inter- 
est in  determining  the  date  of  labor.  The  duration  of  preg- 
nancy is  from  270  to  280  days,  nine  calendar,  or  ten  lunar, 
months,  and  it  dates  from  the  moment  of  conception.  But 
since  the  ovum  retains  its  vitality  for  about  a  week  after  its 
extrusion  from  the  Graafian  follicle,  and  since  the  activity 
of  the  spermatozoa  may  continue  for  several  weeks  after  their 
entrance  into  the  female  genital  tract,  it  is  impossible  to  fix 
accurately  the  date  of  conception  even  in  those  cases  in  which 
there  has  been  but  one  coitus.  It  is  now  believed  by  most 
embryologists  that  the  ovum  is  fertilizable  only  while  it  is 
in  the  Fallopian  tube,  a  period  probably  of  about  seven  days  ; 
if  this  be  true,  it  follows  that  conception  must  occur  within 
a  week  after  ovulation,  although  it  may  be  effected  as  late  as 
two  weeks  after  coitus.  Since  the  ovum  is  usually  discharged 
from  the  ovary  during  the  menstrual  period,  it  is  evident  that 


38  TEXT-BOOK  OF  EMBRYOLOGY. 

the  time  most  favorable  for  conception  is  the  week  following 
menstruation  ;  and  inasmuch  as  the  latter  function  is  sus- 
pended during  pregnancy,  it  is  obvious  that  the  most  reliable 
basis  for  calculating  the  probable  date  of  conception  is  the 
last  menstruation.  The  method  usually  employed  is  to  count 
nine  months  and  seven  days  from  the  first  day  of  the  last 
menstruation.  After  what  has  been  said  it  is  perhaps  need- 
less to  remind  the  reader  that  this  can  furnish  only  approxi- 
mately the  date  of  labor.  In  a  case  where  conception  oc- 
curred a  few  days  prior  to  the  first  omitted  period,  there 
would  be  a  discrepancy  of  several  weeks  between  the  actual, 
and  the  calculated,  termination  of  pregnancy. 

FERTILIZATION. 

Fertilization  is  that  peculiar  union  of  spermatozoon  and 
egg-cell  which  initiates  the  phenomena  resulting  in  the  forma- 
tion of  a  new  individual.  As  implied  in  a  preceding  section, 
impregnation  is  possible  in  the  higher  organisms  only  after 
the  completion  of  maturation,  while  in  others,  as  for  example 
the  maw-worm  of  the  horse,  spermatozoa  enter  the  ovum 
before  the  extrusion  of  the  polar  bodies,  and  thus  one  process 
overlaps  the  other. 

The  more  primitive  method  of  fertilization  is  that  eifected 
without  copulation  of  the  parent  organisms,  or  external  fer- 
tilization ;  this  occurs  in  osseous  fishes,  in  some  amphib- 
ians, and  in  many  invertebrates.  In  these  groups,  both 
ova  and  semen  are  discharged  into  the  water  and  there 
meet.  In  frogs,  however,  there  is  a  quasi-copulation,  the 
male  embracing  the  female  during  the  breeding  season  and 
depositing  semen  upon  the  eggs  as  they  are  evacuated.  In 
all  higher  animals,  internal  fertilization  occurs,  this  being 
effected  by  sexual  congress. 

In  man,  fertilization  normally  occurs  in  the  outer  third  of 
the  Fallopian  tube.  The  semen  having  been  deposited  in 
the  vagina,  or  the  uterus,  or  even  upon  the  vulva,  the  sper- 
matazoa  make  their  way  into  the  oviduct  by  the  vibratile 
motion  of  their  tails.  Meeting  the  ovum,  they  swarm  around 
it,  and  some  of  them  pass  through  the  zona  pellucida  into 


FER  TILIZA  TION. 


39 


the  perivitelHne  space.  It  is  believed  by  many  investigators 
that  the  canals  of  the  zona  constitute  the  avenues  of  entrance 
for  the  spermatozoa.  In  the  rather  firm  egg-envelopes  of 
insects  and  some  fishes,  there  is  a  small  aperture,  the  micro- 
pyle,  through  which  the  spermatozoa  gain  entrance. 

While  many  spermatozoa  may  pass  through  the  zona,  only 
one — that  one  whose  head  first  impinges  against  the  vitelline 
membrane — enters  the  ovum.  Why  others  do  not  or  cannot 
enter  is  unknown ;  possibly  because  the  egg's  power  of  attrac- 
tion is  annulled  (Minot).  Polyspermia,  or  the  penetration  of 
several  spermatozoa,  may  occur,  however,  if  the  ovum  is 
unhealthy;  and  in  some  lower  types  it  is  said  to  be  normal. 

As  the  spermatozoon  is  about  to  strike  the  vitelline  mem- 
brane, the  protoplasm  swells  up  at  the  point  of  contact  into 
the  receptive  pronunence  (Fig.  15).     Through  this  the  sper- 


"■AV." 

Fig.  15. — Portions  of  the  ova  of  Asterias  glacialis,  showing  the  approach  and 
fusion  of  the  spermatozoon  with  the  ovum  (Hertwig) :  a,  fertilizing  male  element; 
6,  elevation  of  protoplasm  of  egg ;  b',  b",  stages  of  fusion  of  the  head  of  the  sper- 
matozoon with  the  ovum. 

matozoon  bores  its  way,  losing  its  tail  in  the  process,  and 
thus  becoming  the  male  pronucleus.  The  female  pronucleus,  it 
will  be  remembered,  lies  in  or  near  the  center  of  the  effff.  The 
two  pronuclei  now  approach  each  other,  and,  upon  meeting, 
fuse  and  temporarily  disappear.  Soon  they  reappear,  and 
now  constitute  the  segmentation-nucleus  or  cleavage-nucleus 
(Fig.  16). 

Since  the  spermatozoon  is  the  metamorphosed  nucleus  of  a 
cell  formed  in  the  testicle,  and  since  the  female  pronucleus  is 
a  part  of  the  nucleus  of  the  ovum,  it  follows  that  the  seg- 


40 


TEXT-BOOK  OF  EMBRYOLOGY. 


mentation-nucleus  consists  of  chromatin  substance  derived 
from  each  parent.  As  this  fact  has  been  thought  to  explain, 
anatomically,  the  offspring's  inheritance  of  both  paternal  and 


Fig.  16.— a,  fertilized  ovum  of  echinus  (Hertwig) :  the  male  (a)  and  the  female 
pronucleus  (6)  are  approaching;  in  B  they  have  almost  fused;  C,  ovum  of  echinus 
after  completion  of  fertilization  (Hertwig) :  s.n.,  segmentation-nucleus. 

maternal  characteristics,  it  has  been  made  the  basis  of  a 
theory  of  heredity  formulated  by  Hertwig  and  independently 
advanced  by  Strasburger. 

Artificial  fertilization,  or  the  bringing  about  of  the  develop- 
ment of  the  ovum  by  artificial  (chemical)  means,  without  the 
participation  of  the  male  element,  has  been  recently  experi- 
mentally effected  with  the  eggs  of  the  sea-urchin  by  Loeb, 
of  Chicago.  These  eggs,  when  first  immersed  for  about  two 
hours  in  a  mixture  of  sea-water  and  a  weak  solution  of 
magnesium  chlorid,  and  then  transferred  to  normal  sea- 
water,  were  found  to  undergo  complete  and  normal  develop- 
ment, producing  perfect  larvae.  This  artificially  induced 
development  differed  from  that  of  the  ordinary  method  only 
in  being  slower. 


CHAPTER    II. 

THE  SEGMENTATION  OF  THE  OVUM  AND  FORMA= 
TION  OF  THE  BLASTODERMIC  VESICLE. 

While  the  fertilized  ovum  is  passing  along  the  Fallopian 
tube  to  the  uterus — a  journey  believed  to  require  seven  or 
eight  days  in  man  ^ — it  undergoes  repeated  segmentation,  or 
cleavage,  becoming  a  more  or  less  globular  mass  of  cells  or 
blastomeres.     This  mass  is  the  mulberry-mass  or  morula. 

The  details  of  the  process  of  division  correspond  closely 
to  those  of  ordinary  indirect  cell-division,  or  karyokinesis. 
The  first  indication  of  approaching  cleavage  is  seen  in  the 
segmentation-nucleus,  just  as,  in  other  cells,  the  sequence  of 
changes  leading  to  cell-division  is  inaugurated  in  the  nucleus. 

The  achromatin-substance  of  the  segmentation-nucleus 
forms  a  nuclear  spindle  in  the  ordinary  manner,  with  a  cen- 
trosome  or  pole- corpuscle  at  each  apex.  The  centrosome 
is  surrounded  by  the  polar  striation  or  attraction-sphere. 
After  the  usual  preliminary  changes,  the  chromatin-substance 
assumes  the  form  of  V-shaped  loops  arranged  around  the 
equator  of  the  spindle  in  such  a  manner  as  to  produce  the 
wreath  or  aster.  Each  chromatin  loop  splits  longitudinally, 
and  the  resulting  halves  of  each  move  to  opposite  poles  of 
the  spindle,  where  they  become  grouped  about  the  pole-cor- 
puscle to  constitute  the  daughter-wreaths  of  the  new  nuclei. 
The  vitellus  now  begins  to  divide,  the  first  step  being  the 
formation  of  an  encircling  groove  on  its  surface ;  this  groove 
deepens  more  and  more  until  finally  division  of  the  cell  is 
complete.  In  like  manner,  each  daughter-cell  divides  into 
two,  and  each  of  these  two  into  other  two,  the  cell-division 
continuing  until  there  results  the  mass  of  cells,  or  morula, 
already  mentioned  (Plate  I,  Fig.  1). 

These  processes  have  been  followed  the  most  accurately  in 

^  Eecent  investigations  by  Peters,  of  Vienna,  upon  an  ovum  of  three  or 
four  days,  already  embedded  in  the  uterine  mucosa,  would  indicate  that  less 
time  than  this  is  occupied  in  traversing  the  oviduct. 

41 


42  TEXT-BOOK  OF  EMBRYOLOGY. 

the  egg  of  the  sea-urchin ;  in  reptilian  eggs,  as  well  as  in 
those  of  the  rabbit  and  other  mammals,  they  have  been 
studied  also  and  have  been  found  to  agree  with  the  former 
in  all  essential  respects.  Certain  modifications  dependent 
upon  the  relations  and  proportions  of  formative-yolk  and 
food-yolk  will  be  pointed  out  hereafter. 

While  no  one  has  seen  the  segmentation  of  the  human 
ovum,  there  is  no  reason  to  suppose  that  it  differs  materially 
from  that  of  other  mammals. 

An  interesting  and  probably  significant  modification  of  the 
method  of  cleavage  as  just  described  has  been  observed  by 
Van  Beneden  in  the  ova  of  the  maw-worm  of  the  horse.  In 
this  case  male  and  female  pronuclei  do  not  fuse  but  merely 
lie  close  together.  At  the  beginning  of  segmentation,  the 
chromatin  of  each  pronucleus  assumes  the  form  of  a  con- 
voluted thread,  which  divides  transversely  into  two  sister- 
threads.  In  this  manner  are  produced  four  loops  of  chro- 
matin, which  become  grouped  around  the  equator  of  the 
nuclear  spindle  just  formed,  and  each  one  of  which  then 
splits  longitudinally  into  two  threads.  In  the  migration  of 
the  segments  that  now  ensues,  each  pair  of  sister-threads 
separates,  one  thread  going  to  one  pole  of  the  spindle,  one 
to  the  other.  Hence,  at  each  pole,  and  taking  part,  there- 
fore, in  the  formation  of  each  new  nucleus,  are  two  male 
and  two  female  threads  of  chromatin.  Thus  the  male  and 
female  pronuclei  contribute  equal  shares  of  chromatin  to 
each  daughter-nucleus. 

Very  suggestive  in  this  connection  is  the  observation  of 
Nussbaum,  that  if  the  same  principle  should  be  found  to 
apply  to  all  subsequent  cell-division,  then  every  cell  of  the 
adult  organism  would  consist  of  equal  amounts  of  material 
from  each  parent. 

Cleavage-planes. — The  direction  of  the  planes  of  cleav- 
age is  determined  by  certain  laws.  The  direction  of  the 
plane  of  the  first  cleavage  bears  a  definite  relation  to  the 
long  axis  of  the  nuclear  spindle,  whose  position,  in  turn,  de- 
pends upon  the  manner  of  distribution  of  the  egg's  proto- 
plasm, its  direction  coinciding  with  the  longest  diameter  of 
an  oval  egg,  but  lying  in  any  diameter  of  a  spherical  one. 


THE  SEGMENTATION  OF  THE   OVUM.  43 

The  first  cleavage-plane  always  cuts  the  axis  of  the  nuclear 
spindle  perpendicularly  at  its  center ;  the  second  bisects  the 
lirst,  also  perpendicularly ;  and  the  third  is  perpendicular  to 
the  two  others,  and  passes  through  the  middle  of  their  axis 
of  intersection. 

Kinds  of  Cleavage. — The  mode  of  cleavage  of  the 
ovum  is  influenced  by  the  relation  of  the  protoplasm  and 
the  deutoplasm  to  each  other,  and  by  their  relative  propor- 
tions. The  classification  of  ova  according  to  their  method 
of  cleavage  is  as  follows : 

1.  Holoblastic  ova  are  those  in  which  segmentation  is  total 
— that  is,  the  entire  ovum  undergoes  division.  If  the  re- 
sulting cells  are  of  equal  size,  there  is  said  to  be 

(a)  Total  equal  cleavage ;  to  this  class  belong  the  alecithal 
ova  of  mammals  (Fig.  4j  and  of  amphioxus,  to  the  segmen- 
tation of  which  the  above  description  may  be  said  to  apply. 
Strictly  speaking,  the  cells  are  not  of  exactly  equal  size, 
those  in  the  region  of  the  vegetative  pole  of  the  egg  being 
slightly  larger  than  those  at  the  animal  pole.  Contrasted 
with  this  is 

(b)  The  total  unequal  cleavage  of  amphibian  ova,  whose 
segments  are  of  unequal  size  (Fig.  17).     These  eggs  being 


Fig.  17. — Diagram  of  the  division  of  the  frog's  egg:  A,  stage  of  the  first  division. 
B.  stage  of  the  second  division.  The  four  segments  of  the  second  stage  of  division 
are  beginning  to  be  divided  by  an  equatorial  furrow  into  eight  segments  :  p,  pig- 
mented surface  of  the  e^^  at  the  animal  pole :  ^r,  the  part  of  the  e^^  which  is 
richer  in  protoplasm ;  d,  the  part  which  is  richer  in  deutoplasm ;  sp,  nuclear 
spindle  (Hertwig). 

telolecithal,  the  lignrer  protoplasmic  animal  pole  is  directed 
upward,  while  the  deutoplasmic  vegetative  pole  is  under- 
neath.    The  inequality  of  the  resulting  segments,  as  well  as 


44 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  direction  of  the  cleavage  planes,  may  be  appreciated  by 
reference  to  Fig.  17,  which  represents  a  frog's  ovum. 

2.  Meroblastic  ova  are  those  in  which  the  segmentation  is 
partial,  division  being  limited  to  the  formative  yolk,  or 
protoplasm. 

(a)  Partial  discoidal  cleavage  is  the  variety  of  meroblastic 
cleavage  that  occurs  in  those  telolecithal  ova  having  a  germ- 
disk  (Fig.  7),  to  which  latter  the  segmentation  is  limited. 
This  method  of  segmentation  is  seen  in  the  eggs  of  birds, 
reptiles,  and  fishes.  In  the  egg  of  the  bird,  which  may  be 
taken  as  a  typical  example,  the  germ-disk,  in  whatever  posi- 
tion the  egg  may  be  placed,  floats  on  the  top  of  the  yolk. 
The  beginning  of  the  first  segmentation  is  indicated  by  a 
furrow  in  the  center  of  the  surface  of  the  germ-disk  (Fig. 
18).       This    furrow    deepens,   cutting    vertically    from    the 


A  B  C 

Fig.  18. — Surface  view  of  the  first  stages  of  cleavage  in  the  heu's  egg  (after 
Coste) :  a,  border  of  tlie  germ-disk ;  6,  vertical  furrow  ;  c,  small  central  segment ;  d, 
large  peripheral  segment. 

upper  to  the  lower  surface  of  the  germ-disk,  dividing  it  into 
two  equal  parts.  Another  groove,  crossing  the  first  at  a 
right  angle,  bisects  each  of  the  two  segments,  and  each  of 
these  is  in  turn  bisected  by  a  radial  furrow,  so  that  the 
germ-disk  now  consists  of  eight  sector-shaped  cells.  Cross 
furrows,  appearing  near  the  center  of  the  disk,  cut  off  the 
apices  of  the  sectors,  adding  small  central  segments.  Cell- 
division  continues  until  the  germ-disk  consists  of  a  flattened 
mass  of  cells,  several  strata  thick,  lying  on  the  surface  of  the 
yolk. 

The  second  method  of  meroblastic  segmentation  is 
(b)  Peripheral  cleavage,  which  occurs  in  the  centrolecithal 
ova  of  arthropods  (Fig.  6).     In  these  eggs,  it  will  be  remem- 


PLATE  I. 


•  1,  2,  3.  Diagrams  illustrating  the  segmentation  of  the  mammalian  ovum  (Allen  Thomson, 
after  van  Beuedeii).  4.  Diagram  illustrating  the  relation  of  the  primary  layers  of  the  blasto- 
derm, the  segmentation-cavity  of  this  stage  corresponding  with  the  archenteron  of  amphioxus 
(Bonnet). 


'  THE  STAGE  OF  THE  BLA8TULA.  45 

bered,  the  nutritive-yolk  is  centrally  placed  and  is  surrounded 
by  the  formative-yolk.  The  segmentation-nucleus  lies  in  the 
center  of  the  nutritive-yolk,  and  in  this  position  undergoes 
division  and  subdivision.  The  new  nuclei  now  migrate  into 
the  peripherally  placed  formative-yolk,  when  the  latter  di- 
vides into  as  many  parts  as  there  are  nuclei,  and  thus  the 
central  un segmented  nutritive-yolk  becomes  enclosed  in  a 
sac  composed  of  small  cells. 

THE   STAGE   OF  THE   BLASTULA. 

t 

Very  soon  there  appears  in  the  interior  of  the  morula  or 
mulberry  mass  referred  to  above,  a  little  fissure-like  space,, 
called  the  cleavage-cavity  or  segmentation-cavity.  When 
this  space  has  increased  somewhat  in  size,  the  germ  is  said 
to  have  reached  the  blastula  stage,  or  the  stage  of  the  blasto- 
dermic vesicle  (Plate  I.,  Fig.  2). 

What  may  perhaps  be  regarded  as  the  primitive  form  of 
the  blastula  is  that  of  the  lancelet,  or  amphioxus  lanceolatus^ 
one  of  the  lowest  vertebrates,  a  fish-like  animal  several  inches 
in  length  inhabiting  the  Mediterranean  Sea.  The  blastula  in 
this  case  is  a  simple  sac  composed  of  cells  which  surround 
the  cleavage-cavity  as  a  single  layer  (Fig.  21,  A).  The  cells 
in  the  region  of  the  vegetative  pole  are  larger  and  more  tur- 
bid, because  more  deutoplasmic,  than  those  at  the  animal 
pole,  as  shown  in  the  same  figure. 

The  mammalian  blastula  is  a  hollow  sphere,  whose  wall  is 
a  layer  of  cells,  the  outer  cell-mass,  and  into  whose  central 
space,  the  cleavage-cavity,  projects  an  irregular  mass  of  gran- 
ular cells  constituting  the  inner  cell-mass  (Plate  L,  Fig.  2). 
The  cleavage-cavity  contains  an  albuminous  fluid.  It  is 
during  this  stage  that  the  germ,  in  the  case  of  mammals, 
reaches  the  uterus.  As  a  peculiarity  of  the  mammalian 
ovum,  the  blastula  now  increases  greatly  in  size,  the  cleav- 
age-cavity becoming  dilated.  The  zona  pellucida,  which 
still  surrounds  the  ovum,  is  by  this  time  quite  attenuated, 
and  is  called  the  prochorion.  The  outer  cell-mass,  likewise 
much  thinned-out,  constitutes  Rauber's  layer  (Plate  I.,  Fig. 
4).     The  significance  of  the  cells  of  Rauber's  layer  is  un- 


46 


TEXT-BOOK  OF  EMBRYOLOGY. 


known.  After  a  time  they  disappear,  probably  by  disinte- 
gration (Kolliker).  The  form  of  the  blastula  of  amphibians 
and  of  the  Sauropsida  (birds  and  reptiles)  is  greatly  modified 
by  the  relatively  abundant  nutritive  yolk  with  which  their 

ova  are  endowed.  An 
amphibian  ovum  in  the 
blastula  stage  is  shown 
in  Fig.  19.  It  will  be 
seen  that  its  walls  con- 
sist of  several  layers  of 
cells,  and  the  cleavage- 
cavity  is  encroached 
upon  to  a  considerable 
extent  by  the  large  and 
abundant  cells  of  the 
vegetative  pole,  which 
are  especially  rich  in 
deutoplasm.  In  the 
eggs  of  birds  and  rep- 
tiles— that  is,  in  the  telolecithal  eggs  that  undergo  partial 
discoidal  segmentation — the  blastula  form  is  so  markedly 
modified  as  to  be  scarcely  recognizable.  In  this  case,  as 
shown  in  Fig.  20,  the  cleavage-cavity  is  a  narrow   fissure 


Fig.  19. — Blastula  of  triton  tseniatus :  Jh,  seg- 
mentation-cavity ;  rz,  marginal  zone ;  dz,  cells 
with  abundant  yolk  (Hertwig). 


Fig.  20.— Median  section  through  a  germ-disk  of  pristiurus  in  the  blastula  stage 
(after  Ruekert) :  B,  cavity  of  the  blastula ;  kz,  segmented  germ  ;  dk,  finely  granular 
yolk  with  yolk-nuclei. 

whose  roof  is  the  germ-disk,  and  whose  floor  is  the  unseg- 
mented  nutritive-yolk,  which  latter  corresponds  therefore  to 
the  large  vegetative  cells  forming  the  floor  of  the  amphibian 
G^g  shown  in  Fig.  19. 


CHAPTEE    III. 

THE  QERM=LAYERS  AND  THE   PRIMITIVE  STREAK. 
THE  STAGE  OF  THE  QASTRULA. 

By  the  conversion  of  the  one-layered  germ,  the  blastula, 
into  the  germ  with  two  layers,  the  gastrula  stage  is  attained. 
The  gastrula,  in  its  typical  form,  consists  of  two  layers  of 
cells  surrounding  a  central  cavity,  which  latter  communicates 
with  the  exterior  by  means  of  a  small  aperture,  the  blasto- 
pore. The  cavity  is  the  archenteron  or  coelenteron  or  intes- 
tino-body  cavity.  The  outer  layer  of  cells  is  the  ectoderm 
or  epiblast;  the  inner  layer  is  the  entoderm  or  hypoblast. 
This  form  of  the  germ  is  seen  in  holoblastic  invertebrate,  as 
well  as  in  some  vertebrate  ova,  and  is  typically  exemplified 
in  the  development  of  the  amphioxus.  The  blastula  of  this 
animal  is  a  simple  sac,  the  wall  of  which  is  a  single  layer  of 
epithelial  cells  surrounding  the  cleavage-cavity  (Fig.  21,  A). 
By  a  pushing-in  of  the  vegetative  cells,  the  cleavage-cavity 
is  encroached  upon  and  finally  is  completely  obliterated,  being 
replaced  by  the  archenteron  (Fig.  21,  C).  From  this  it  is 
obvious  that  gastrulation  occurs  here  by  a  simple  process  of 
invagination.  In  ova  with  a  large  amount  of  food-yolk,  as 
in  those  of  frogs,  birds,  and  fishes,  the  process  is  modified 
and  complicated  by  this  condition. 

According  to  the  so-called  gastrula  theory  of  Haeckel,  all 
metazoa — that  is,  multicellular  animals  as  distinguished  from 
protozoa,  or  ufaicellular  organisms — pass  through  a  typical 
gastrula  stage  in  the  course  of  their  development. 

It  has  been  held  as  a  general  principle  that  the  higher 
animals  during  their  development  repeat,  to  a  greater  or  less 
extent,  the  embryonic  or  the  larval  forms  of  the  lower  mem- 
bers of  the  group  to  which  they  belong.  Huxley  has  pointed 
out  the  morphological  identity  of  the  adult  form  of  the  coelen- 

47 


48 


TEXT-BOOK  OF  EMBRYOLOGY, 


terata  with  the  two-layered  gastrula.  This  principle  is  one 
of  importance  as  applying  to  the  developmental  history  of 
the  various  organs. 

It  must  not  be  understood,  however,  that  we  find  in  mam- 
mals such  a  gastrula  as  that  of  amphioxus ;  some  embryolo- 
gists,  indeed,  do  not  employ  the  term  gastrula  in  connection 


Fig.  21.— Gastrulation  of  amphioxus  (modified  from  Hatschek).  A.  Blastula: 
az,  animal  cells ;  vz,  vegetative  cells ;  Jh,  cleavage-cavity.  B.  Beginning  invagina- 
tion of  vegetative  pole.  C.  Gastrula  stage,  the  invagination  of  the  vegetative  cells 
being  complete:  ak,  outer  germ-layer;  ik,  inner  germ-layer;  ud,  archenteron;  u, 
blastopore. 

with   mammalian  development,  but  call  the  germ  a  blasto- 
dermic vesicle  even  after  it  has  become  two-layered. 

The  mammalian  "gastrula"  (Plate  L,  Fig.  4)  is  a  vesicle 
whose  wall,  for  the  most  part,  is  a  single  layer  of  flattened 
cells,  known  as  Rauber's  layer ;  this  layer  has  been  referred 
to  above  as  the  attenuated  outer  cell-mass  of  the  blastula. 
Throughout  a  limited  area,  the  germ-wall  is  composed  of  two 
other  layers  of  cells  in  addition  to  the  layer  of  Rauber.  The 
inner  of  these  layers,  composed  of  flattened  cells,  is  the  ento- 
derm ;  the  outer,  lying  next  to  Rauber's  layer,  and  consisting 
of  cubical  cells,  is  the  ectoderm.  At  a  later  stage  both  layers 
take  part  in  the  formation  of  the  entire  wall  of  the  vesicle. 


THE  STAGE   OF  THE  GASTBULA  49 

The  ectoderm  and  the  entoderm  are  produced  from  the 
inner  cell-plate  of  the  blastula.  This  mass  gradually  flattens, 
becoming  at  first  lens-shaped  ;  spreading  out  peripherally  on 
the  inner  surface  of  Rauber's  layer,  it  is  at  length  differen- 
tiated into  the  two  primary  germ-layers,  the  ectoderm  and 
the  entoderm. 

The  cavity  of  the  vesicle  is  the  archenteron,  or  ccelenteron. 
As  previously  stated,  the  process  of  gastrulation  and  the  form 
of  the  gastrula  are  modified  in  the  case  of  ova  possessing  a 
large  proportion  of  deutoplasm.  In  the  case  of  the  frog, 
for  example,  as  well  as  in  other  amphibians,  the  blastula  has 
the  form  shown  in  Fig.  19.  By  an  invagination  of  the  blas- 
tula-wall  at  the  place  of  transition  from  the  animal  cells  to 
the  vegetative  cells,  all  of  the  latter  and  a  part  of  the  former 
are  carried  into  the  interior  of  the  blastula  to  form  the  lining 
of  the  archenteron  (Fig.  22).    Compare  this  with  the  amphi- 


FiG.  22.— Sagittal  section  through  an  egg  of  triton  (after  the  end  of  gastrulation) ; 
ak,  outer  germ-layer ;  ik,  inner  germ-layer ;  dz,  yolk-cells ;  dl  and  vl,  dorsal  and 
ventral  lips  of  the  ccelenteron ;  ud,  ccelenteron ;  d,  vitelline  plug ;  mk,  middle 
germ-layer  (Hertwig). 


oxus  gastrula  as  shown  in  Fig.  21.  In  the  bird's  egg,  the 
form  of  whose  gastrula  is  shown  in  Fig.  23,  an  infolding 
or  invagination  occurs,  as  in  the  frog's  eg^,  at  the  place  of 
transition  from  the  animal  cells  to  the  vegetative  cells,  or,  in 
other  words,  at  the  margin  of  the  germ-disk.  The  gastrula 
thus  formed  is  represented  in  Fig.  23.     Its  archenteron  is  a 


50 


TEXT-BOOK  OF  EMBRYOLOGY. 


narrow  fissure,  and  its  blastopore,  situated  at  tlie  posterior 
margin  of  the  germ-disk,  is  exceedingly  small. 

The  l^mbryonal  Area. — Upon  the  surface  of  the  germ 
at  the  beginning  of  gastrulation — that  is,  at  about  the  fifth 
day  of  development  in  the  case  of  the  rabbit's  germ — there  is 


Fig.  23.— Longitudinal  section  through  the  germ-disk  of  a  fertilized  unincubated 
egg  of  the  nightingale  (after  Duval) :  ak,  outer,  ik,  inner  germ-layer ;  ud,  ccelen- 
teron;  vl,  anterior,  hi,  posterior  lip  of  the  blastopore  (crescentic  groove). 

a  round  whitish  spot,  the  embryonal  area.  Its  position  corre- 
sponds to  that  formerly  held  by  the  inner  cell-plate  of  the 
blastula,  as  shown  in  Plate  I.,  Fig.  2.  It  is  only  in  this 
region  that  the  wall  of  the  vesicle  is,  at  this  particular  stage, 
composed  of  more  than  a  single  layer  of  cells,  the  ectoderm 
and  the  entoderm  not  extending  much,  if  at  all,  beyond  its 
periphery. 

The  embryonal  area,  soon  becoming  oval  (Fig.  24)  and, 
later,  pear-shaped,  exhibits,  at  its  posterior  margin,  a  trans- 


FiG.  24.— Blastula  of  the  rabbit  seven  days  old  without  the  outer  egg-mem- 
branes. Length  4.4  mm.  (after  Kolliker).  Magnified  ten  diameters.  Seen  in  A 
from  above,  in  B  from  the  side :  ag,  embryonic  spot  (area  embryonalis) ;  ge,  the  line 
up  to  which  the  blastula  is  two-layered. 

verse  thickening  called  the  terminal  ridge,  which  is  believed 
to  be  the  anterior  lip  of  the  blastopore.  It  may  be  not  amiss 
to  say  that  the  terms  anterior  and  posterior  are  used  Avith 


THE  STAGE  OF  THE  GASTRULA. 


51 


reference  to  the  future  body,  the  narrow  end  of  the  area  em- 
bryonalis  corresponding  to  the  posterior  pole  or  caudal  ex- 
tremity of  the  fetus. 

In  the  chick's  egg,  the  embryonic  area  (Fig.  25),  or  em- 
bryonic shield,  appears  while  the  egg  is  yet  in  the  oviduct. 


Fig.  25.— Two  germ-disks  of  hen's  egg  in  the  first  hours  of  incubation  (after 
Keller) :  df,  area  opaca ;  hf,  area  pellucida ;  s,  crescent ;  sk,  crescent-knob ;  es,  em- 
bryonic shield ;  "pr,  primitive  groove. 

Its  embryonic  crescent  corresponds  to  the  raamalian  terminal 
ridge.  Segmentation  being  limited  to  the  germ-disk  in  the 
chick's  egg,  the  resulting  blastoderm,  which  is  not  a  vesicle, 
but  a  flattened  mass  (Fig.  23)  composed  of  several  layers  of 
cells,  rests  by  its  margin  upon  the  partially  liquefied  yolk. 
The  central  region  of  the  blastoderm,  which  overlies  the 
liquefied  portion  of  the  yolk,  from  its  translucence  is  known 
as  the  area  pellucida  (Fig.  25),  while  the  dark  opaque  rim, 
resting  upon  the  yolk  is  the  area  opaca.  The  inner  rim  of 
the  area  opaca  is  the  area  vasculosa.  These  regions  are 
observed  also  in  the  mammalian  egg. 

It  is  in  the  embryonal  area  alone  that  the  body  of  the 
embryo  is  developed ;  the  other  parts  of  the  germ  produce 
extra-embryonic  structures,  such  as  the  amnion,  the  yolk- 
sac,  etc. 

Partial  longitudinal  division  of  the  embryonic  area  dur- 
ing development  results  in  the  production  of  some  form  of 
double  monster ;  its  complete  cleavage  gives  rise  to  homologous 
or  homogeneous  twins,  which  are  twins  of  the  same  sex  and 
of  almost  absolutely  identical  structure.  Ordinary  twins  are 
developed  from  separate  ova,  which  may  or  may  not  have 
come  from  the  same  ovary. 

The  Primitive  Streak. — The  primitive  streak  is  a  linear 


52 


TEXT-BOOK  OF  EMBRYOLOGY. 


median  marking  lying  in  the  long  axis  of  the  embryonal 
area  and  containing  a  median  furrow,  the  primitive  groove 
(Fig.  26).    A  transverse  section  through  the  primitive  streak 


Primitive 
streak. 


Fig.  26.— Embryonic  area  of  rabbit-embryo  (E.  v.  Beneden) :  primitive  streak  begin- 
ning in  cell-proliferation  known  as  the  "  node  of  Hansen." 

(Figs.  27  and  28)  shows  that  this  surface-marking  is  pro- 
duced by  a  thickening  of  the  ectoderm  along  the  median 
line,  owing  to  a  proliferation  of  cells  from  its  under  side. 
The  length  of  the  streak  is  about  two-thirds  of  that  of  the 
embryonal  area.  In  the  rabbit's  ovum  it  is  seen  at  about 
the  seventh  day ;  in  the  human  germ  the  time  of  its  appear- 


FiG.  27.— Section  across  the  primitive  streak  of  rabbit-embryo  (Kolliker):  ec, 
ectoderm;  ax.  ec,  axial  ectoderm  undergoing  proliferation,  as  shown  by  karyo- 
kinetic  figures  (fc);  en^,  entoderm  ;  m,  mesoderm. 


ance  is  not  known,  but  is  probably  about  the  twelfth  or 
thirteenth  day.  In  the  case  of  such  a  gastrula  as  that  of  the 
amphioxus  (Fig.  21),  the  lips  of  the  blastopore  approach  each 


THE  STAGE  OF  THE  OASTBULA. 


53 


other  and  fuse  in  a  line  corresponding  to  the  median  longi- 
tudinal axis  of  the  future  embryonic  area,  the  fusion  or  con- 
crescence beginning  at  the  anterior  extremity  of  this  line 
and  proceeding  toward  its  caudal  end.  The  surface-marking 
produced  by  the  apposition  and  partial  union  of  the  blasto- 
poric  lips  was  called  the  primitive  streak,  and  its  median 
furrow  was  known  as  the  primitive  groove,  long  before  their 
true  significance  was  appreciated.  Since  the  edge  of  the 
blastopore  marks  the  place  of  transition  from  the  entoderm 
to  the  ectoderm  (Fig.  21),  the  two  germ-layers  after  the 


Primitive  groove. 


Beginning 
amnion  fold. 


Ectoderm. 


Visceral  layer 
of  mesoderm. 


Entoderm. 


Fig.  28.— Transverse  section  of  the  embryonic  area  of  a  fourteen-and-a-half-day 
ovum  of  sheep  (Bonnet). 


union  of  the  edges  of  this  opening  are  in  intimate  association 
under  the  primitive  streak,  as  shown  in  Fig.  28. 

Morphologically  the  primitive  streak  of  the  higher  verte- 
brates is  regarded  as  the  fused  and  extended  blastopore  of 
lower  types.  The  terminal  ridge  of  the  mammalian  embry- 
onic area,  as  well  as  the  crescent  of  the  embryonic  shield  of 
avian  and  reptilian  eggs,  represents,  as  stated  above,  the 
anterior  lip  of  the  blastopore.  Since  the  embryonal  area  is 
increasing  in  circumference  while  the  lips  of  the  blastopore 
are  undergoing  union  or  concrescence,  the  transversely  di- 
rected terminal  ridge,  which  lies  at  the  posterior  edge  of  the 
embryonal  area,  and  which  remains  a  fixed  point,  becomes  a 


54  TEXT-BOOK   OF  EMBRYOLOGY. 

longitudinal  marking,  and  this  marking  or  primitive  streak 
comes  to  lie,  therefore,  behind  the  site  of  the  blastopore. 
Reference  to  Duval's  diagram  (Fig.  29)  will  make  this  clear. 


/  '1  !(( 


I  >  !  I 


Mil 


Fig.  29. — Diagram  elucidating  the  formation  of  the  primitive  groove  (after 
Duval).  The  increasing  size  of  the  germ-disk  in  the  course  of  the  development  is 
indicated  by  dotted  circular  lines.  The  heavy  lines  represent  the  crescentic 
groove  and  the  primitive  groove  which  arises  from  it  by  the  fusion  of  the  edges 
of  the  crescent. 

After  the  development  of  the  primitive  streak,  there  is 
seen,  in  the  median  line  of  the  embryonal  area,  anterior  to 
the  streak,  another  marking,  the  head-process  of  the  primitive 
streak.  This  is  almost  identical  with  the  primitive  axis  of 
Minot,  which  that  investigator  describes  as  a  median  band  of 
cells  connected  with  the  entoderm  and  extending  forward 
from  the  blastopore. 

Hansen's  node  is  an  accumulation  of  cells  on  the  under 
surface  of  the  ectoderm  at  the  anterior  end  of  the  primitive 
streak.  It  is  important  because  of  its  relation  to  the  neuren- 
teric  canal,  which  will  be  described  later. 

Although  the  primitive  streak  and  blastopore  play  no  part 
in  the  later  stages  of  development,  it  is  worthy  of  note  that 
the  former  lies  in  the  line  of  the  longitudinal  axis  of  the 
future  body,  and  that  the  position  of  the  blastopore  marks 
the  posterior  or  caudal  end  of  the  embryo. 

The  Development  of  the  Mesoderm. — The  mesoderm 
or  mesoblast  is  a  structure  composed  of  several  layers  of  cells 
lying  between  the  ectoderm  and  the  entoderm.  It  is  earliest 
formed  in  the  vicinity  of  the  front  end  of  the  primitive 
streak,  the  position  formerly  held  by  the  blastopore.  From 
this  point  it  grows  laterally  and  posteriorly  and,  later,  anteri- 
orly as  well.  It  is  not,  however,  until  other  important 
changes  have  taken  place  that  it  extends  completely  around 
the  germ. 


THE  STAGE  OF  THE  GASTBULA.  55 

The  terms  gastral  mesoderm  and  ^peristomal  mesoderm 
are  used  to  designate  respectively  that  portion  developing 
from  the  region  of  the  head-process  of  the  primitive  streak 
and  that  portion  growing  from  the  region  of  the  blastopore. 

Concerning  the  origin  of  the  mesoderm  much  diiference  of 
opinion  prevails.  The  simpler  and  more  primitive  method 
is  seen  in  the  amphioxus,  in  which  it  develops  as  two  evagina- 
tions  from  the  dorsal  w^all  of  the  archenteron,  one  on  each 
side  of  the  mid-line.  These  entodermic  folds,  containing 
each  a  cavity,  the  enteroccel,  grow  out  laterally  between  the 
inner  and  the  outer  germ-layers.  By  transverse  constriction, 
each  fold  divides  into  a  series  of  segments,  the  somites,  which 
lie  on  either  side  of  the  median  line  from  the  head-end  to  the 
tail-end  of  the  embryo.  Each  somite  divides  into  a  dorsal 
part,  the  "  protovertebra,"  and  a  ventral  part,  the  lateral 
plate.  By  the  fusion  of  the  lateral  plates  of  each  side  their 
several  cavities  become  one,  the  body-cavity  or  coelom. 

The  origin  of  the  middle  germ-layer  in  higher  vertebrates 
is  far  less  clearly  made  out.  Some  investigators  hold  that  it 
arises  in  essentially  the  same  manner  as  does  that  of  amphi- 
oxus— that  is,  by  evagination  or  outfolding  of  the  entoderm 
bounding  the  coelenteron ;  the  investigations,  however,  of 
Bonnet  and  of  Duval  respectively  upon  sheep  and  chick 
embryos,  point  to  a  different  conclusion.  Bonnet's  observa- 
tions show  that  the  mesodermic  tissue,  starting  from  Hensen's 
node,  grows  out  laterally  between  the  ectoderm  and  the  ento- 
derm, and  that  at  some  distance  from  the  median  line  of.  the 
embryonic  area  there  is  a  delamination  or  splitting-olf  of 
cells  from  the  entoderm  ;  and,  further,  that  these  two  primi- 
tive areas  grow  toward  each  other  and  unite  to  form  one 
continuous  sheet  of  mesoderm.  It  may  be  said,  therefore, 
that  the  mesoderm  originates  from  a  double  source,  chiefly 
from  the  entoderm.,  hid  also  from  the  ectoderm,  since  the  cells 
giving  rise  to  the  part  that  grows  from  the  region  of  Hen- 
sen's  node  are  ectodermic.  A  section  of  the  germ  transverse 
to  the  long  axis  of  the  embryonic  area  (Figs.  27  and  28) 
shows  the  mesoderm  to  be  a  distinct  and  independent  layer, 
sharply  defined  from  the  other  germ-layers  everywhere  except 


56 


TEXT-BOOK  OF  EMBRYOLOGY. 


in  the  region  of  the  mid-line,  in  which  position  the  three 
layers  are  so  closely  related  as  to  constitute  one  structure. 
The  mesoderm  does  not  extend  completely  around  the  germ 
at  this  stage,  being  deficient  on  the  side  opposite  the  embry- 
onic area. 

The  mesoderm,  after  its  formation,  grows  by  the  prolifera- 
tion of  its  own  cells,  independently  of  the  ectoderm  and  the 
entoderm. 

If  the  expansion  of  the  mesoderm,  as  indicated  by  the 
surface  appearance  of  the  germ  (Fig.  30),  be  noted,  it  will 


\   ^ 


Fig.  30.— Diagrammatic  surface  view  of  rabbit's  ovum  of  205  hours  (after  Tour- 
neuxV  The  darkly  shaded  area  indicates  the  extent  of  the  mesoderm,  a,  Periph- 
eral limit  of  area  opaca ;  h,  of  area  pellucida;  c,  of  parietal  zone ;  d,  of  stem-zone ; 
/,  Hensen's  node ;  g,  proamnion. 

be  seen  that  at  first  it  is  present  throughout  a  pear-shaped 
area  whose  narrow  end  is  directed  forward.  Somewhat  later, 
two  wing-like  expansions  grow  forward  from  the  front  end 
of  this  area  (Fig.  31);  these  wings,  meeting  at  their  tips, 
enclose  a  space,  the  proamnion,  which  is  devoid  of  mesoderm. 
Referring  again  to  the  transverse  section  (Fig.  28),  it  is 
evident  that  the  middle  germ-layer  in  the  vicinity  of  the 
median  line  is  composed  of  a  somewhat  irregular  mass  of 
cells,  while  farther  away  it  constitutes  a  lamina  on  each  side. 


THE  STAGE  OF  THE  GASTRVLA. 


57 


As  development  advances,  these  two  portions  become  more 
differentiated  from  each  other,  although  they  are  not  entirely- 
separated  until  much  later.  The  thick  mass  adjacent  to  the 
median  line  is  the  vertebral  plate,  or  primitive  segment  plate, 
or  paraxial  mesoderm ;  the  more  flattened  lateral  portion  is 
the  lateral  plate.  The  mesoderm  at  this  stage,  therefore, 
consists  of  four  parts — the  two  paraxial  masses,  lying  one  on 
each  side  of  the  median  line,  and  extending  from  the  head- 


\ 


Fig.  31.— Diagrammatic  surface  view  of  rabbit's  ovum  of  211  hours  (after  Tour- 
neux).  The  darkly  shaded  area  indicates  the  extent  of  the  mesoderm.  1,  Periph- 
eral limit  of  area  opaca ;  2,  of  area  pellucida ;  3,  of  parietal  zone ;  4,  of  stem-zone ; 
6,  Hensen's  node  ;  7,  proamnion. 

end  to  the  tail-end  of  the  embryonal  area,  and  the  two  lateral 
plates,  situated  upon  the  outer  sides  of  the  paraxial  columns. 

Each  primitive  segment  plate  undergoes  transverse  division 
into  a  number  of  irregularly  cubical  masses,  the  mesoblastic 
somites,  or  primitive  segments,  often  improperly  called  the 
proto vertebrae.  The  presence  and  position  of  the  primitive 
segments  are  indicated  by  transverse  parallel  lines  on  the 
surface  of  the  germ,  w^iich  constitute  a  series  on  either  side 
of  the  primitive  streak  and  its  head-process  (Figs.  31  and 
37).  The  formation  of  the  somites  begins  at  the  cephalic  end 
of  the  embryo  and  progresses  tail  ward. 

The  lateral  plate  of  the  mesoderm  splits  into  two  lamellae, 


58  TEXT-BOOK  OF  EMBRYOLOGY. 

of  which  the  outer  or  parietal  layer  is  the  somatic  mesoderm, 
and  the  inner  or  visceral  layer  is  the  splanchnic  mesoderm. 
The  somatic  mesoderm  unites  with  the  ectoderm,  forming  the 
somatopleure ;  the  splanchnic  mesoderm  unites  with  the  ento- 
derm, forming  the  splanchnopleure.  The  fissure-like  cavity 
between  the  somatopleure  and  the  splanchnopleure  is  the 
coelom,  or  body-cavity,  or  pleuroperitoneal  cavity  (Fig.  36). 
The  great  serous  cavities  of  the  adult  body — pleural,  peri- 
cardial, and  peritoneal — are  later  subdivisions  of  the  coe- 
lom. 

The  mesoderraic  cells  bounding  the  body-cavity  become 
flattened  and  endothelioid  in  character,  and  constitute  the 
mesothelium ;  from  them  are  descended  the  various  endothe- 
lial cells  lining  the  serous  cavities  of  the  mature  organism. 
According  to  some  authorities,  among  whom  Hertwig  may 
be  especially  mentioned,  there  develop  from  the  mesothelium 
at  an  early  stage  certain  cells  whose  particular  function  is 
the  formation  of  the  different  kinds  of  connective  tissue, 
such  as  bone,  cartilage,  fibrous  tissue,  etc. ;  these  elements 
are  often  distinguished  as  mesenchymal  cells,  or  collectively, 
as  mesenchyme.  According  to  this  classification,  the  impor- 
tance of  which  is  insisted  upon  by  Minot,  the  mesenchyme 
includes  all  the  mesodermic  tissue  except  the  flattened  cells, 
the  mesothelium,  lining  the  body-cavity. "^ 

His  claims  a  double  origin  for  the  mesoderm.  He  main- 
tains that  the  mesothelium  and  the  smooth  musculature  of 
the  body  are  of  intra-embryonic  origin,  and  these  structures 
he  terms  the  archiblast ;  while  all  other  parts  of  the  meso- 
derm, which  he  designates  the  parablast,  have,  in  his  opinion, 
an  extra-embryonic  source,  being  derived  possibly  from  the 
granulosa  cells  of  the  ovary.  These  views  are  not  shared, 
however,  by  the  majority  of  embryologists. 

The  Derivatives  of  the  Germ-layers. — From  the 
three  primary  germ-layers  are  developed  the  various  tissues 
and  organs  of  the  body   by  metamorphoses  which  may  be 

'  Minot  holds  with  Goette  that  the  mesenchymal  cells  are  the  product 
of  the  mesothelium.  Hertwig;  maintains  that  the  mesenchyma  arises  from 
all  the  other  germ-layers  by  the  emigration  of  isolated  cells. 


THE  STAGE  OF  THE  GASTBULA.  59 

referred  to  the  two  fundamental  processes  of  specialization, 
or  the  adaptation  of  structure  to  function,  and  of  unequal 
growth,  which  latter  results  in  the  formation  of  folds,  ridges, 
and  constrictions. 

From  the  ectoderm  are  produced  : — 

The  epidermis  and  its  appendages,  including  the  nails,  the 
epithelium  of  the  sebaceous  and  sweat-glands  and  their  invol- 
untary muscles,  the  hair,  and  the  epithelium  of  the  mammary 
glands. 

The  infoldings  of  the  epidermis,  including  the  epithelium 
of  the  mouth,  with  the  enamel  of  the  teeth,  the  epithelium 
of  the  salivary  glands,  and  the  anterior  lobe  of  the  pituitary 
body  : 

The  epithelium  of  the  nasal  tract  with  its  glands  and  com- 
municating cavities : 

The  epithelial  lining  of  the  external  auditory  canal,  includ- 
ing the  outer  stratum  of  the  merabrana  tympani  : 

The  lining  of  the  anus  and  of  the  anterior  part  of  the 
urethra  : 

The  epithelium  of  the  conjunctiva  and  of  the  anterior  part 
of  the  cornea,  the  crystalline  lens. 

The  spinal  eord,  the  brain  with  its  outgrowths,  including 
the  optic  nerve,  the  retina,  and  the  posterior  lobe  of  the 
pituitary  body. 

The  epithelium  of  the  internal  ear. 

From  the  entoderm  are  produced  : — 

The  epithelium  of  the  respiratory  tract. 

The  epithelium  of  the  digestive  tract,  from  the  back  part 
of  the  pharynx  to  the  anus,  including  its  associated  glands, 
the  liver,  and  the  pancreas. 

The  epithelial  parts  of  the  middle  ear  and  of  the  Eustachian 
tube. 

The  epithelium  of  the  thymus  and  thyroid  bodies. 

The  epithelium  of  the  bladder,  and  of  the  first  part  of  the 
male  urethra,  and  of  the  entire  female  urethra. 


60  TEXT-BOOK  OF  EMBRYOLOGY. 

From  the  mesoderm  are  developed  : — 

Connective  tissue  in  all  its  modified  forms,  such  as  bone, 
dentine,  cartilage,  lymph,  blood,  fibrous  and  areolar  tissue. 

Muscular  tissue. 

All  endothelial  cells,  as  of  joint-cavities,  bursal  sacs, 
lymph-sacs,  blood-vessels,  pericardium  and  endocardium, 
pleura,  and  peritoneum. 

The  spleen. 

The  kidney  and  the  ureter. 

The  testicle  and  its  system  of  excretory  ducts. 

The  ovary,  the  Fallopian  tube,  the  uterus,  and  the  vagina. 

From  the  foregoing  tabulation  it  may  be  seen  that,  gener- 
ally speaking,  all  epithelial  structures  originate  from  either 
the  ectoderm  or  the  entoderm,  the  notable  exception  to  this 
rule  being  that  the  epithelium  of  the  sexual  glands  and  their 
ducts,  and  also  that  of  the  kidney  and  of  the  ureter,  proceed 
from  the  mesoderm. 


CHAPTER    IV. 

THE  BEGINNING  DIFFERENTIATION  OF  THE  EM= 
BRYO;  THE  NEURAL  CANAL;  THE  CHORDA 
DORSALIS;    THE  MESOBLASTIC   S0MITE5. 

The  germ,  in  the  stages  thus  far  considered,  has  the  form 
of  a  hollow  sphere  or  vesicle.  It  will  be  seen,  in  following 
the  further  history  of  development,  that  the  layers  of  cells 
constituting  the  walls  of  the  vesicle  give  rise  to  the  alterations 
of  external  form  and  to  the  rudiments  of  the  various  organs 
of  later  stages  by  processes  which,  though  seemingly  com- 
plex, are  referable  to  certain  simple  fundamental  principles. 
It  is,  namely,  in  the  unequal  growth  of  different  parts  of  the 
germ,  in  outfoldings  and  infoldings,  and  in  the  furrowing  and 
constricting-off  of  parts,  as  well  as  in  the  adaptation  of  struct- 
ure to  function,  that  we  find  an  explanation  of  the  various 
developmental  processes. 

The  first  indication  of  the  formation  of  the  embryo  and  of 
its  differentiation  from  the  parts  of  the  germ  that  are  destined 
to  produce,  wholly  or  in  part,  the  several  extra-embryonic 
structures,  is  the  marking  out  of  the  embryonic  area  by  the 
thickening  of  the  cells  of  the  vesicle-wall  in  a  definitely  cir- 
cumscribed region.  The  structures  designated  as  extra-em- 
bryonic are  the  umbilical  vesicle,  the  amnion,  the  allantois,  and 
the  fetal  part  of  the  placenta.  The  development  of  these  and 
the  production  of  the  external  form  of  the  body  of  the  em- 
bryo will  be  considered  in  the  next  chapter. 

The  primitive  streak  and  its  head-process  have  been  already 
described.  After  their  appearance  the  further  evolution  of 
the  embryonic  body  is  closely  associated  with  three  funda- 
mentally important  processes — namely,  the  formation  of  the 
neural  canal,  of  the  chorda  dorsalis,  and  of  the  mesoblastic 
somites. 

61 


62 


TEXT-BOOK  OF  EMBRYOLOGY. 


The  Neural  or  Medullary  Canal. — The  neural  canal 
is  an  elongated  tube  lying  beneath  the  ectoderm  in  the  me- 
dian longitudinal  axis  of  the  embryonic  body,  its  position 
corresponding  to  that  of  the  future  spinal  canal.  Its  walls 
are  composed  of  cylindrical  epithelial  cells. 

To  follow  the  development  of  the  medullary  canal,  it  is 
necessary  to  study  the  surface  appearance  of  the  ovum  at  the 
stage  when  the  mesoderm  is  beginning  to  grow  out  from  the 
region  of  the  head-process  of  the  primitive  streak.  Upon 
the  surface  of  such  a  germ  (Fig.  26),  one  may  see  the  primi- 
tive streak  and,  in  front  of  it,  also  in  the  median  line  of  the 
embryonic  area,  the  head-process  of  the  primitive  streak.  The 
ectodermic  cells  overlying  the  head-process  thicken  so  as  to 


Medullary  

plates.  117^  [■      :.j| 


Primitive  streak 
and  groove. 


Fig.  32.— Surface  view  of  area  pellucida  of  an  eighteen-hour  chick-embryo 
(Balfour). 


become  columnar,  while  those  on  each  side  of  it  become  flat- 
tened. This  diiferentiation  results  in  the  production  of  a 
relatively  thick  axial  plate  of  ectoderm,  the  medullary  plate, 
which  is  present  at  the  beginning  of  the  eighth  day  in  the 
rabbit's  germ,  and  in  the  human  germ  at  about  the  fourteenth 


THE  NEURAL   OR  MEDULLARY  CANAL.  63 

Afnnion. 


Mt  toderm. 


Visceral 
mesoderm. 


Pleuropericar-        Pericardial 
dial  cavity.  J>lates. 

Fig.  33. — Transverse  section  of  a  sixteen-and-a-half-day  sheep-embryo  (Bonnet), 


Extetision 
of  Coelom. 


day.     Almost  as  soon  as  the  plate  is  formed,  its  lateral  and 
anterior  edges  begin  to  curl  up,  producing  the  medullary  fur- 


MeduUary 
furroiv. 


Uncleft 
Ectoderm.        mesoderm.         Amnion. 


Visceral 
tnesoderm. 


NotocJinrd  Somite.     Gut  entoderm. 

Fig.  34.— Transverse  section  of  a  sixteen-and-a-half-day  sheep-embryo  possessing 
six  somites  (Bonnet). 

row  or  groove  (Figs.  32,  33,  and  34).     The  curling  margins 
of  the   plate  carry  with  them,  as   they   rise,   the  adjacent 


64 


TEXT-BOOK   OF  EMBRYOLOGY. 


thinner  ectoderm ;  these  projections  constitute  the  medullary- 
folds.  A  surface  view  shows  the  medullary  folds  to  be 
continuous  with  each  other  in  front,  while  their  posterior 
ends  are  separated  and  embrace  between  them  the  front  end 
of  the  primitive  streak  (Fig.  32).  Since  the  formation  of 
these  structures  is  always  more  advanced  in  the  anterior  part 
of  the  embryonic  area,  their  posterior  extremities  are  not 
sharply  defined  but  fade  away  (Fig.  32).  The  edges  of  the 
medullary  plate  continue  to  curl  until  they  meet,  when  they 
unite,  forming  the  medullary  or  neural  canal  (Figs.  35  and 


Ectoderm. 


Amnion. 


Parietal 
mesoderm. 


Cell-mass  for 
Wolffian  body, 

Coelom. 

Mesothelium. 

Primitive 
endothelium.. 

Visceral 
mesoderm. 


Notochord. 


Fig.  35.— Transverse  section  of  a  fifteen-and-a-half-day  sheep-embryo  possessing 
seven  somites  (Bonnet). 


36).  The  medullary  folds  and  plate  continuing  to  advance 
toward  the  tail-end  of  the  embryonic  area,  and  the  closure 
of  the  tube  taking  place  from  before  backward,  the  entire 
primitive  streak  is  made  to  disappear  by  being  included  within 
the  neural  tube. 

The  medullary  folds  having  grown  toward  each  other  a 
short  time  before  the  union  of  the  edges  of  the  medullary 
plate  now  unite  over  the  partially  formed  neural  tube.  By 
the  growth  of  the  medullary  folds  and  their  subsequent 
coalescence,  the  completed  neural  tube  comes  to  lie  under  the 
surface  ectoderm,  its  connection  with  which  is  afterward  lost^ 


THE  NOTOCHOBD   OR  CHORDA  DOBSALIS. 


65 


It  is  apparent,  therefore,  that  the  neural  tube  is  a  structure 
whose  walls  are  composed  of  ectodermic  cells,  and  that  it  has 
originated  from  the  ectoderm  by  what  may  be  called  a  process 
of  infolding. 

The  medullary  canal  is  the  fundament  of  the  entire  adult 
nervous  system.  The  first  step  in  the  conversion  of  a  struct- 
ure so  simple  into  one  so  complex  consists  in  the  dilatation  of 
the  cephalic  end  of  the  neural  tube  and  the  subsequent  division 
of  this  dilated  extremity  into  three  iraperfestly  separated  com- 
partments, named  respectively  the  fore-brain,  the  mid-brain, 
and  the  hind-brain  vesicles.     It  is  by  the  multiplication  and 


Ajciai  zone.  .  Neural  canal. 


JJiteral  plates  for 
body-walls. 


L-ateml  plates /or 
rut-tract. 


Lateral  zone. 


Cavity  within  somite. 


Parietal  mesoderrtt. 


Pleuroperitoneal 

cavity. 


Vitelline  vein. 
Fig.  36.— TransTerse  section  of  a  seventeen-and-a-half-day  sheep-embryo  (Bonnet). 


specialization  of  the  cells  composing  the  walls  of  the  medul- 
lary tube  that  the  cerebrospinal  axis  is  produced,  the  brain- 
vesicles  giving  rise  to  the  brain-mass,  while  the  remainder  of 
the  tube  produces  the  spinal  cord.  Approximately  one-half 
of  the  length  of  the  tube  is  devoted  to  the  formation  of  the 
brain,  the  other  half  forming  the  spinal  cord. 

The  neural  tube  closes  first  in  the  future  cervical  region, 
the  cephalic  part  of  the  canal  remaining  open  for  a  time. 
From  the  neck  region  the  closure  of  the  tube  progresses 
toward  either  end  of  the  embryo. 

The  Notochord  or  Chorda  Dorsalis. — The  notochord 
is  a  solid  cylindrical  column  of  cells  lying  parallel  with  the 
medullary  tube,  on  the  dorsal  side  of  the  archenteric  cavity. 


66  TEXT-BOOK  OF  EMBRYOLOGY. 

Its  position  is  that  of  a  line  passing  through  the  centers  of 
the  bodies  of  the  future  vertebrae.  The  development  of  the 
chorda  occurs  at  the  same  time  as  that  of  the  neural  tube, 
and  in  a  very  similar  manner.  A  thickening  of  the  cells  of 
the  entoderm  in  a  longitudinal  line  extending  along  the 
dorsal  aspect  of  the  coelenteron  produces  the  chordal  plate. 
Along  either  edge  of  the  chordal  plate  a  small  fold  of  ento- 
derm projects  ventralward.  By  the  curling  around  of  the 
edges  of  the  chordal  plate,  the  latter  becomes  a  solid  cylinder 
of  cells,  which  is  separated  from  the  entoderm  proper  by  the 
union  of  the  chordal  folds,  as  shown  in  Figs.  35  and  36. 

The  appearance  of  the  notochord  is  the  first  indication  of 
the  axis  of  the  embryo,  since  around  it  the  permanent  spinal 
column  is  built  up.  The  relative  size  of  the  chorda  is  less 
in  the  higher  vertebrates  than  in  the  lower  members  of  this 
group.  It  is  one  of  the  distinctive  features  of  a  vertebrated 
animal. 

The  chorda  is  essentially  an  embryonic  structure,  since  it 
gives  rise  to  no  adult  organ.  Its  only  representative  in 
postnatal  life  is  the  pulpy  substance  in  the  centers  of  the 
intervertebral  disks.  It  is  a  permanent  structure  in  one 
vertebrate  only,  the  amphioxus.  In  this  animal  it  is  the 
representative  of  the  spinal  column  of  higher  vertebrates. 
The  notochord  aifords  another  illustration  of  the  principle 
that  higher  organisms  repeat,  in  their  development,  the 
structure  of  the  lower  members  of  the  group  to  which  they 
belong. 

The  Neurenteric  Canal. — The  neurenteric  canal  is 
closely  associated  with  the  development  of  the  medullary 
canal  and  with  the  disappearance  of  the  ])rimitive  groove. 
We  have  learned  that  the  blastopore  is  the  orifice  through 
wKich  the  coelenteron  opens  to  the  exterior,  and  also  that  in 
birds  and  mammals  the  position  of  the  blastopore,  as  indi- 
cated by  the  presence  of  the  terminal  ridge,  corresponds  to 
the  anterior  end  of  the  primitive  streak,  and  therefore  of  the 
primitive  groove.  Reference  to  Fig.  32  will  show  that  the 
medullary  folds  have  extended  so  far  posteriorly  that  they 
embrace  between  them  the  primitive  groove ;  therefore  when 


THE  SOMITES  OR  PRIMITIVE  SEGMENTS.  67 

they  unite  to  form  the  neural  canal,  the  primitive  streak  falls 
within  its  limits. 

In  a  gastrula  with  an  open  blastopore,  such  as  that  of  the 
amphioxus  and  those  of  amphibians,  the  blastopore  is  in- 
cluded between  the  medullary  folds,  and,  after  the  completion 
of  the  neural  canal,  it  constitutes  an  avenue  of  communica- 
tion between  the  latter  and  the  coelenteron  or  primitive  enteric 
cavity;  this  communication  is  the  neurenteric  canal.  In 
mammals,  as  also  in  birds,  reptiles,  and  selachians,  classes  in 
which  the  primitive  streak  is  the  representative  of  the  closed 
blastopore,  a  small  canal  is  found  at  the  anterior  end  of  the 
primitive  groove,  passing  through  Hensen's  node,  and  open- 
ing into  the  coelenteron.  With  the  covering  in  of  the  primi- 
tive groove  by  the  medullary  folds,  this  canal  becomes  the 
neurenteric  canal.  According  to  Graf  Spec,  a  neurenteric 
canal  is  found  in  the  human  embryo,  as  well  as  in  the  groups 
above  mentioned.  The  canal  is  a  temporary  structure  and 
gives  rise  to  no  organ  of  the  adult. 

The  Somites  or  Primitive  Segments.— The  meso- 
blastic  somites  are  cuboidal  masses  of  cells,  arranged  in  two 
parallel  rows,  one  on  each  side  of  the  notochord,  extending 
the  entire  length  of  the  body  of  the  embryo.  They  are 
sometimes  called  x>rotovertebrce,  but  this  term  if  used  at  all 
should  be  restricted  to  a  subdivision  of  them  that  appears 
later. 

The  development  of  the  somites  was  incidentally  referred 
to  in  the  description  of  the  mesoderm.  As  mentioned  in  that 
connection,  the  paraxial  plates  of  mesoderm,  lying  as  parallel 
longitudinal  columns,  one  on  each  side  of  the  notochord, 
break  up,  each  one  into  its  corresponding  series  of  primitive 
segments.  The  division  throughout  the  entire  length  of  the 
body  takes  place  not  simultaneously,  but  consecutively,  begin- 
ning at  the  head-end. 

The  segmentation  of  the  axial  mesoderm  is  indicated  by 
certain  surface  markings.  The  surface  of  the  embryonal 
area,  at  the  stage  when  the  primitive  streak  and  the  medul- 
lary groove  are  present,  shows  a  dark  zone  on  either  side  of 
the  median  line,  the  so-called   stem-zone,  which  marks  the 


68 


TEXT-BOOK  OF  EMBRYOLOGY. 


limits  of  the  axial  plate  of  mesoderm  (Fig.  37) ;  the  position 
of  the  lateral  plates  is  indicated  by  the  peripheral  lighter 
parietal  zone.  The  stem-zone  soon  exhibits,  on  each  side  of 
the  primitive  streak  and  medullary  groove,  a  series  of  parallel 
transverse  lines,  produced  by  the  transverse  furrowing  of  the 
axial  plates,  preparatory  to  their  division  into  the  primitive 
segments.  The  first  pair  of  somites  is  formed  in  the  future 
cervical  region,  before  the  medullary  folds  have  united  to 
form  the  neural  tube,  and  when  the  primitive  streak  is  yet 
present.     After  the  appearance  of  the  first  pair,  the  forma- 

\<2^  TTtfl  /t,A 

^risi^^^wii,^    _„__      ,   _         _ 

-■III  llln"  -1^  „ 

^^      I  '"iiymi, ,|^,|  I,  ,  nil, mil 


Fig.  37.— Rabbit  embryo  of  the  ninth  day,  seen  from  the  dorsal  side  (after 
Kolliker).  Magnified  21  diameters.  The  stem-zone  {stz)  and  the  parietal  zone  {pz) 
are  to  be  distinguished.  In  the  former  8  pairs  of  primitive  segments  have  been 
established  at  the  side  of  the  chorda  and  neutral  tube ;  ap,  area  pellucida ;  rf, 
medullary  groove ;  vh,  fore-brain ;  ab,  eye- vesicle  ;  mh,  mid-brain ;  hb,  hind-brain ; 
j<w,  primitive  segment:  ste,  stem-zone ;  pz,  parietal  zone;  h,  heart;  p/i,  pericardial 
part  of  the  body-cavity ;  vd,  margin  of  the  entrance  to  the  head-gut  (vordere 
Darmpforte),  seen  through  the  overlying  structures;  of,  amniotic  fold;  vo,  vena 
omphalomesenterica. 

tion  of  other  segments  proceeds  headward  and  tailward.  In 
selachians  the  number  of  head-segments  has  been  shown  to 
be  nine ;  in  higher  vertebrates  the  number  is  possibly  less. 
The  trunk-segments  are  added  in  regular  order  from  the 
neck-region  to  the  tail-end  of  the  embryo. 

The  first  somites  appear  on  the  eighth  day  in  the  rabbit, 
and  between  the  twentieth  and  twenty-second  hours  in  the 
chick.  While  they  are  forming,  the  neural  canal  is  closing, 
the  notochord  is  diiferentiating  from  the  entoderm,  and  the 
lateral  plates  of  mesoderm  are  splitting  to  form  the  body- 
cavity  or  coelom. 


THE  SOMITES  OR  PRIMITIVE  SEGMENTS.  69 

In  structure  the  primitive  segments  of  lower  vertebrates 
consist  of  columnar  cells  arranged  around  a  central  cavity 
(Figs.  34  and  36).  The  cavity,  in  the  amphioxus,  communi- 
cates for  a  time  with  the  coelenteron,  since  the  segments 
are  in  this  case  developed  as  entodermic  evaginations ;  in 
selachians,  the  method  of  formation  of  whose  primitive  seg- 
ments may  be  regarded  as  the  primitive  method  for  ver- 
tebrates, the  cavity  is  for  a  time  in  communication  with  the 
body-cavity,  since  the  segments  in  these  animals  develop  as 
if  by  evagination  from  the  dorsal  side  of  the  mesoderm  after 
it  has  separated  into  its  parietal  and  visceral  layers  and  before 
it  has  divided  into  the  axial  and  lateral  plates.  The  size  of 
the  cavity  is  quite  variable;  in  some  cases,  as  in  the  Amniota, 
it  is  almost  if  not  entirely  obliterated  by  the  encroachment 
of  the  cells  of  the  walls  of  the  somite. 

Belonging  to  the  somite,  though  not  apparent  on  the  sur- 
face, is  a  mass  of  cells  which  connects,  for  some  time,  the 
somite  proper  with  the  lateral  plate  (Fig.  36).  This  is 
known  as  the  intermediate  cell-mass  or  middle  plate.  Later, 
the  separation  of  these  is  effected,  the  mesial  part  of  the 
somite  being  the  myotome,  the  intermediate  cell-mass 
becoming  the  nephrotome.  Each  one  of  these  parts 
contains  a  cavity,  that  of  the  myotome  being  called  the 
myocoel.  From  the  inner,  mesial  side  of  the  myotome, 
embryonic  connective-tissue  cells  (mesenchyme)  develop, 
constituting  the  sclerotome,  or  skeletogenous  tissue.  The 
sclerotomes,  made  up  of  loosely-arranged  embryonal  con- 
nective tissue,  grow  around  the  medullary  canal  and  chorda 
dorsalis,  spreading  out  and  fusing  with  each  other.  Subse- 
quently this  tissue  produces  the  vertebral  column  and  its 
associated  ligamentous  and  cartilaginous  structures.  The 
outer  part  of  the  myotome,  sometimes  called  the  cutis  plate, 
gives  rise  to  the  corium  of  the  skin  of  the  trunk.  The  re- 
maining part  of  the  myotome,  that  situated  dorsolaterally, 
constitutes  the  muscle-plate  or  myotome  proper ;  it  gives  rise 
to  the  voluntary  musculature  of  the  trunk. 

The  segmentation  of  the  body  of  the  embryo  is  an  embryo- 
logical  process  of  great  significance. 


70  TEXT-BOOK  OF  EMBRYOLOGY. 

The  segmented  condition  is  common  to  the  developmental 
stage  of  all  true  vertebrates,  and  in  some  invertebrates  it 
persists  throughout  adult  life.  The  development  of  the 
axial  skeleton  and  of  the  muscular  system,  it  will  be  seen 
later,  bears  an  important  relation  to  the  process  of  segmen- 
tation, as  does  also  the  evolution  of  the  genito-urinary 
system. 

Upon  reflection,  it  will  be  seen  that  in  the  region  of  the 
embryo  corresponding  to  the  future  neck  and  trunk,  the 
segmentation  aifects  only  the  dorsal  part  of  the  body,  while 
the  ventral  mesoderm,  the  so-called  lateral  plate,  which  con- 
tains the  coelom,  remains  unsegmented.  On  the  other  hand, 
in  the  head-region,  the  segmentation  is  both  dorsal  and  ven- 
tral, the  former  being  in  series  with  the  trunk-segments, 
while  the  latter,  affecting  the  ventral  mesoderm,  and  there- 
fore also,  in  the  corresponding  region,  the  coelom,  produces 
the  structures  known  as  the  visceral  arches  (see  Chapter  VII.). 

The  relation  of  the  primitive  segments  to  the  differentia- 
tion of  the  skeleton  and  of  the  musculature  of  the  trunk,  and 
also  of  the  visceral  arches  to  the  muscles  of  the  jaws,  will  be 
considered  in  subsequent  chapters. 


CHAPTEE    Y. 

THE  FORMATION  OF  THE  BODY=WALL,  OF  THE 
INTESTINAL  CANAL,  AND  OF  THE  FETAL 
MEMBRANES. 


The  formation  of  the  fetal  membranes  occurs  coincidentally 
with  the  production  of  the  external  form  of  the  body  of  the 
embryo.  These  changes  mark  the  division  of  the  hollow 
sphere  or  vesicle  of  which  the  germ  consists  up  to  this  stage 
into  two  essentially  diiferent  parts — namely,  the  embryonic 
body  and  the  fetal  appendages,  the  latter  of  which  are  destined 
for  the  nutrition  and  protection  of  the  growing  embryo. 
Although  the  several  processes  by  which  are  produced  the 
different  parts  of  the  embryo  and  its  various  appendages  go 
on  simultaneously,  it  is  necessary,  for  the  sake  of  clearness, 
to  consider  successively  the  development  of  each  structure 
from  its  inception  to  its  completion. 

THE  FORMATION  OF  THE   BODY=WALL  AND  OF  THE 
INTESTINAL  CANAL  OF  THE   EMBRYO. 

In  the  stages  of  development  thus  far  considered,  the  part 
of  the  ovum  that  is  to  become  the  embryo — that  is,  the 
embryonic  area — is  represented  by  a  localized  thickening  of 
the  wall  of  the  blastodermic  vesicle,  of  the  shape  and  relative 
size  shown  in  Fig.  24,  which  presents  a  surface  view  of  the 
germ.  On  each  side  of  the  embryonic  axis,  represented  by 
the  notochord,  is  the  paraxial  mass  of  mesoderm,  which  has 
undergone  partial  segmentation  to  form  the  somites ;  on  the 
distal  side  of  the  paraxial  column,  the  mesoderm  has  split 
into  the  somatic  or  parietal,  and  the  splanchnic  or  visceral 
lamellae,  between  which  is  the  body-cavity  or  coelom.  The 
cavity  of  the  germ  until  the  occurrence  of  the  transforma- 

71 


72  TEXT-BOOK  OF  EMBRYOLOGY. 

tions  about  to  be  described  is  one  undivided  compartment 
which  is  bounded  by  splanchnopleure ;  and  a  conspicuous 
feature  of  the  changes  under  consideration  is  the  division  of 
this  cavity  into  two  by  the  folding  in  of  the  splanchnopleure 
conij)osing  its  walls. 

The  first  indication  of  the  foldings  that  lead  to  the  differ- 
entiation of  the  embryo  from  the  fetal  appendages  is  seen 
upon  the  surface  of  the  germ  at  a  very  early  stage.  A  sur- 
face view  of  the  germ — in  the  case  of  the  chick  on  the  first 
day  of  incubation — shows,  at  what  becomes  the  head-end  of 
the  embryonic  area,  a  transverse  crescentic  groove,  with  its 
concavity  looking  backward  (Fig.  32) ;  a  similar  groove  is 
seen  at  the  opposite  extremity  of  the  area,  and  also  one  at 
each  lateral  margin.  These  marginal  grooves  are  depres- 
sions in  the  somatopleure.  The  elevated  outer  edges  of  the 
grooves  form  folds  of  somatopleure,  designated  respectively 
the  head-fold,  the  tail-fold,  and  the  lateral  folds  of  the  amnion. 
As  these  marginal  grooves  increase  in  length  they  meet  each 
other  and  now  constitute  one  continuous  furrow,  which  encir- 
cles the  embryonic  area  ;  its  outer  elevated  edge  is  the  amnion- 
fold.  This  furrow,  which  may  be  called  an  inverted  fold 
composed  of  splanchnopleure  and  somatopleure,  progressively 
deepens  and  at  the  same  time  its  bottom  is  carried  inward 
toward  a  point  vertically  under  the  central  region  of  the 
embryonic  area ;  that  is,  a  fold  composed  of  somatopleure 
and  splanchnopleure  grows  from  all  parts  of  the  periphery 
of  the  embryonic  area  toward  the  point  indicated  above,  a 
point  which  corresponds  to  the  site  of  the  future  umbilicus. 
By  the  ingrowth  of  the  edges  of  the  fold,  the  cavity  of  the 
archenteron  is  more  and  more  constricted  (Plate  II.,  Figs. 
2  and  3),  until  finally,  with  the  completion  of  the  infolding, 
it  becomes  divided  into  two  parts  of  unequal  size  ;  the  smaller 
of  these  spaces  is  the  gut-tract,  or  intestinal  canal  of  the  em- 
bryo, while  the  larger  is  the  yolk-sac  or  umbilical  vesicle. 
The  constricted  canal  through  which  the  gut-tract  commu- 
nicates with  the  yolk-sac  is  the  vitelline  duct  (Plate  II., 
Figs.  4  and  5). 

While  the  splanchnopleuric  layer  of  the  ingrowing  fold 


THE  FORMATION  OF  THE  BODY-WALL.  73 

thus  outlines  and  forms  the  walls  of  the  intestinal  canal,  the 
soraatopleuric  layer,  which  accompanies  it,  constitutes  the 
lateral  and  ventral  body-walls  of  the  embryo.  During  the 
progress  of  this  infolding  of  the  splanchnopleure  and  the 
somatopleure,  the  part  of  the  latter  membrane  that  forms 
the  outer  wall  of  the  groove  becomes  lifted  up  to  constitute 
the  amnion-fold  (Plate  II.,  Fig.  3) ;  by  the  continued  upward 
growth  of  this  amnion-fold  and  the  simultaneous  settling 
down  of  the  embryo  upon  the  yolk-sac,  the  margins  of  the 
fold  come  to  lie  above  the  embryonic  body,  and,  approaching 
€ach  other,  they  fuse  over  its  back,  in  this  manner  enclosing 
it  in  a  cavity.  It  is  obvious  that  the  fold  just  described  is  a 
double  layer  of  somatopleure.  After  the  union  of  its  edges, 
the  two  layers  become  completely  separated,  the  inner  one 
constituting  the  amnion,  while  the  outer  layer  is  the  false 
amnion,  or  serosa,  or  chorion  (Plate  II.,  Figs.  4-6). 

Since  the  infolding  of  the  splanchnopleure  begins  at  the 
periphery  of  the  much  elongated  embryonic  area,  the  result- 
ing gut-tract  has  the  form  of  a  straight  tube  extending  from 
the  head-end  to  the  tail-end  of  the  embryo  (Plate  III.). 
When  the  caudal  and  the  cephalic  portions  of  the  splanchno- 
pleuric  fold  have  advanced  but  a  comparatively  short  dis- 
tance, in  consequence  of  which  the  communication  between 
the  gut-tract  and  the  umbilical  vesicle  is  still  widely  open 
as  shown  in  Plate  II.,  Fig.  5,  there  is  a  cul-de-sac  or  pocket 
formed  of  splanchnopleure  at  the  head-end  of  the  embryo 
and  a  similar  one  at  its  tail-end;  these  recesses  are  respec- 
tively the  foregut  and  the  hindgut,  the  orifices  of  which  are 
designated  the  intestinal  portals.  At  this  particular  stage, 
therefore,  the  cavity  of  the  gut-tract  is  incompletely  closed 
oft  from  that  of  the  umbilical  vesicle. 

It  is  evident  that  the  gut-tract,  being  k  tubular  cavity 
enclosed  by  splanchnopleure,  is  lined  with  entodermal  cells; 
this  simple  straight  tube  develops  subsequently  into  the  adult 
intestinal  canal  and  its  associated  glandular  apparatus. 

It  has  already  been  pointed  out  that  the  layer  of  somato- 
pleure which  is  folded  under  the  embrvonic  area  in  company 
with  the  splanchnopleure  constitutes  the  lateral  and  the  ven- 


74  TEXT-BOOK  OF  EMBRYOLOGY. 

tral  walls  of  the  body  of  the  embryo.  The  fold  continues 
to  advance  from  each  side  and  from  each  end,  and  its 
edges  come  together  and  fuse  in  the  median  line  of  the 
ventral  surface  of  the  body.^  At  one  place,  however,  fusion 
of  the  edges  of  the  fold  does  not  occur ;  this  region  corre- 
sponds to  the  umbilicus  and  is  often  designated  the  dermal 
navel.  Here  the  part  of  the  somatopleure  that  forms  the 
body-wall  is  continuous  with  that  part  of  this  membrane 
which  constitutes  the  amnion  (Plate  II.,  Fig.  6).  By  the 
infolding  of  the  somatopleure  the  body-cavity  or  pleuro- 
peritoneal  space  becomes  divided  into  an  intra-embryonic 
and  an  extra-embryonic  portion,  the  two  communicating  for  a 
time  through  the  small  annular  space  that  encircles  the 
proximal  end  of  the  vitelline  duct;  this  is  represented  in  the 
accompanying  figures. 

By  this  simple  process  of  folding,  associated  with  the 
unequal  growth  of  different  parts,  the  leaf-like  fundament 
constituted  by  the  embryonic  area  is  differentiated  into  the 
body  of  the  embryo ;  the  ventral  portion  of  this  body  now 
consists  of  two  tubes,  one  within  the  other,  of  which  the 
smaller,  bounded  by  the  splanchnopleure,  is  the  intestinal 
canal,  and  the  larger,  enclosed  by  the  somatopleure,  is  the 
body-cavity,  the  walls  of  which  are  the  walls  of  the  body  of  the 
embryo.  In  the  dorsal  region  is  a  third  tube,  the  medullary 
canal ;  between  it  and  the  dorsal  wall  of  the  intestine  is  the 
notochord,  on  each  side  of  which  are  the  somites  (Fig.  35). 
The  further  evolution  of  this  body  and  the  differentiation  of 
its  various  organs  and  systems  will  be  described  in  subse- 
quent sections. 

THE  AMNION. 

The  amnion  is  a  membranous  fluid-filled  sac,  which  sur- 
rounds the  fetus  of  certain  groups  of  vertebrate  animals 
during  a  part  of  their  period  of  development.     In  man,  it  is 

1  Failure  of  union  of  the  somatopleuric  folds  in  the  median  line  of  the 
thorax  produces  the  deformity  known  as  cleft  sternum  ;  while  lack  of  fusion 
of  the  lateral  halves  of  the  abdominal  wall  results  in  an  extra-abdominal 
position  of  the  intestines,  or,  if  in  lesser  degree,  in  exstrophy  of  the  bladder. 


THE  AMNION.  75 

found  as  early  as  the  fourteenth  day/  before  the  medullary 
groove  has  closed  to  form  the  neural  canal ;  it  attains  its 
maximum  size  by  the  end  of  the  sixth  month  and  persists 
until  the  end  of  gestation.  It  constitutes  a  loose  envelope 
for  the  fetus,  being  attached  to  the  abdominal  wall  of  the 
latter  at  the  margins  of  the  umbilicus,  and  loosely  enveloping 
the  umbilical  cord  (see  Plate  III.,  Fig.  2). 

An  amnion  is  found  in  birds,  reptiles,  and  mammals,  these 
groups  being  classed  together  as  Amniota,  while  fishes  and 
amphibians,  which  are  without  an  amnion,  constitute  the 
class  Anamnia. 

The  first  indication  of  the  growth  of  the  amnion  is  apparent 
at  a  comparatively  early  stage  of  development.     A  surface- 
view  of  the  blastodermic  vesicle  of  the  first  day  of  incuba- 
tion in  the  case  of  the  chick,  or  of  about  the  eighth  day  of 
development  in  the  rabbit,  shows  a  curved  line  or  marking 
at  the  anterior  edge  of  the  embryonic  area  (Fig.  32) ;  this  is 
the  anterior  marginal  groove,  in  front  of  which  is  another 
marking,  the  head-fold  of  the  amnion.     Very  soon  the  lateral 
and  posterior  marginal  grooves  appear  at  the  sides  and  poste- 
rior edge  respectively  of  the  embryonic  area;  the  outer  ele- 
vated edges  of  these  marginal  grooves  constitute  the  lateral 
folds  and  the  tail-fold  of  the  amnion.     The  grooves  and  folds 
increase  in  length  in  each  direction  until  they  meet,  when 
they  form  one  continuous  furrow,  which  circumscribes  the 
embryonic  area,  and  the  outer  elevated  edge  of  which  is  the 
amnion  fold.     The  groove  involves  both  the  somatopleure 
and  the  splanchnopleure,  constituting  the  inverted  fold  of 
these  two  structures  that  grows  in  to  form  the  body-wall  and 
the  wall  of  the  gut-tract,  while  the  amnion  fold  is  composed 
of  somatopleure  alone  (Plate  II.).     This  separation  of  the 
somatopleure  and  the  splanchnopleure  enlarges  the  extra- 
embryonic portion   of  the  body-cavity.      The  amnion   fold 
continues  to  grow  upward,  and  finally  its  edges  meet  and  fuse 
over  the  back  of  the  embryo,  the  line  of  union  being  the  am- 
niotic suture ;  the  suture  closes  first  at  the  head-end  of  the 
embryo  and  last  at  the  tail-end.  After  the  union  of  the  edges 

^  Recently  it  has  been  found  complete  in  an  ovum  estimated  to  be  four 
davs  old. 


76  TEXT-BOOK  OF  EMBRYOLOGY.       , 

of  the  fold,  its  inner  layer,  consisting  of  ectoderm  and  parie- 
tal mesoderm,  separates  from  the  outer  layer  to  constitute  the 
true  amnion,  whose  enclosed  space  is  the  amniotic  cavity ;  the 
outer  layer,  which  is  merely  a  part  of  the  general  soraato- 
pleure,  is  the  false  amnion  or  serosa.  It  is  apparent  from 
this  description  that  the  amniotic  cavity  is  lined  with  ecto- 
dermal epithelium  and  that  its  walls  consist  of  somatopleure 
— that  is,  of  ectoderm  and  parietal  mesoderm. 

While  the  amnion  fold  is  growing  upward,  the  embryonic 
area — now  undergoing  differentiation  into  the  embryonic 
body — is  sinking  down  upon  the  yolk-sac.  The  amnion  fold 
does  not  grow  uniformly  in  all  parts  of  its  periphery.  The 
head-fold  is  produced  first  and  constitutes  a  cap  or  hood  cov- 
ering the  head  of  the  embryo,  which  is  forming  simultaneously 
by  the  ventrad  growth  of  the  somatopleure  at  the  bottom  of 
the  marginal  groove.  It  .is  only  after  the  development  of  the 
head-fold  is  well  advanced  that  the  lateral,  and,  later,  the 
caudal,  portions  of  the  amnion-fold  grow  up  to  meet  it.  The 
head-fold  is,  for  a  time,  destitute  of  mesodermic  tissue,  since 
it  corresponds  to  that  region  of  the  wall  of  the  blastodermic 
vesicle  described  on  page  56  as  the  proamnion. 

The  amnion  of  man  presents  an  important  variation 
from  that  of  all  other  Amniota,  since  the  inner  layer  of  the 
amnion-fold  does  not  entirely  sever  its  connection  with  the 
outer  layer,  but  remains  attached  to  it  over  the  caudal  pole 
of  the  embryo.  In  consequence  of  this  attachment  the  true 
amnion  is  connected  with  the  false  amnion,  and  since  the  true 
amnion  is  continuous  with  the  body-wall  of  the  embryo,  the 
caudal  end  of  the  embryonic  body  is  attached  to  the  false 
amnion  or  chorion  by  a  mass  of  tissue  called  the  allantoic 
stalk  or  belly-stalk,  as  seen  in  Fig.  38.  The  relation  of  the 
belly-stalk  to  the  development  of  the  allantois  will  be  pointed 
out  hereafter. 

The  space  within  the  amnion — the  amniotic  cavity — is  filled 
with  the  amniotic  fluid  or  liquor  amnii. 

The  amnion  at  first  envelops  only  the  sides  and  dorsum  of 
the  embryonic  body,  occupying  the  upper  part  of  the  cavity 
enclosed  by  the  chorion,  as  shown  in  Plate  II.,  Figs.  5  and 


THE  AMNION.  77 

6  ;  the  groove,  or  furrow,  however,  of  which  the  amnion  fold 
is  the  peripheral  or  outer  elevated  edge,  becomes  deeper,  and 
the  bottom  of  the  groove  is  carried  toward  the  middle  of  the 
future  ventral  surface  of  the  embryo,  its  ventrad  growth  con- 
tinuing until  it  reaches  the  position  of  the  future  umbilicus 
(Plate  II. :  Fig.  4,  transverse  section ;  Figs.  5  and  6,  longi- 
tudinal section).  The  layer  of  somatopleure  constituting  the 
inner  wall  of  the  groove — that  is,  on  the  side  toward  the 
embryonic  area — becomes  the  lateral  and  ventral  walls  of 
the  body  of  the  embryo,  as  described  above ;  in  this  manner 
is  effected  the  transition  from  the  flattened  or  layer-like  em- 
bryonic area  to  the  definite  form  of  the  embryonic  body. 
The  ventral  body-wall  is  continuous  at  the  margins  of  the 
umbilicus  with  the  amnion,  since  the  somatopleure,  forming 
the  outer  boundary  of  the  original  groove,  is  a  part  of  that 
membrane.  After  its  completion,  therefore,  the  amnion  en- 
velops the  body  of  the  embryo  on  every  side,  lying  closely 
applied  to  it,  since  the  amniotic  cavity  is  at  first  very  small. 
With  the  progress  of  development  and  the  increase  of  the 
fluid  the  amnion  requires  more  room,  until,  in  the  third 
month,  it  fills  out  the  entire  space  within  the  chorion,  with 
the  inner  surface  of  which  it  acquires  a  loose  connection. 
The  umbilical  vesicle  and  the  allantois  have  meanwhile 
undergone  regression.  The  walls  of  the  amniotic  sac  con- 
tain contractile  fibers ;  it  is  to  these  that  the  rhythmical  con- 
tractions observed  in  the  amnion  are  due.  Its  lining  is  at 
first  a  single  layer  of  flattened  epithelial  cells ;  at  the  fourth 
month  the  cells  are  cubical  for  the  most  part,  but  to  some 
extent  columnar. 

The  liquor  amnii  is  a  watery  fluid  having  a  specific  gravity 
of  1.007,  and  containing  about  1  per  cent,  of  solids  (albumin, 
urea,  and  grape  sugar).  The  origin  of  the  fluid  is  believed 
to  be  in  the  blood  of  the  mother,  the  liquid  portion  of  which 
transudes  into  the  amniotic  cavity.  The  amniotic  fluid  in- 
creases in  quantity  until  the  sixth  month  of  pregnancy  ;  from 
this  time  until  the  close  of  gestation  it  generally  diminishes 
about  one  half.  A  pathological  excess  of  the  fluid  constitutes 
the  condition  of  hydramnios. 

The  function  of  the  amniotic  fluid  is  two-fold  ;  it  serves  as 


78  TEXT-BOOK  OF  EMBRYOLOGY. 

a  buflPer  for  the  fetus,  protecting  it  from  mechanical  violence, 
and  it  supplies  the  fetal  tissues  with  water,  since  portions  of 
it  are  from  time  to  time  swallowed.  Evidence  that  the  fetus 
swallows  the  fluid  is  afforded  by  direct  observation  of  chicken 
embryos,  and  by  the  presence  of  epidermal  cells,  hairs,  and 
fatty  matter  in  the  fetal  alimentary  canal.  After  the  devel- 
opment of  the  bladder,  the  urine  of  the  fetus  is  from  time  to 
time  evacuated  into  the  amniotic  cavity. 

The  epidermis  of  the  child  in  utero  is  protected  against 
maceration  in  the  amniotic  fluid  by  the  presence  of  a  fatty 
coating,  the  vernix  caseosa,  which  is  a  modified  sebaceous 
secretion. 

At  the  end  of  pregnancy,  the  amnion  is  loosely  united 
with  the  chorion  and  the  deciduse ;  during  birth  it  ruptures, 
and  its  fluid  escapes. 

THE  YOLK=SAC. 

The  yolk-sac,  or  umbilical  vesicle,  as  seen  in  the  higher 
vertebrates,  is  a  capacious  sac  attached  by  a  narrow  pedicle, 
the  vitelline  duct,  to  the  ventral  surface  of  the  embryonic 
intestinal  canal,  the  duct  passing  through  the  umbilical  aper- 
ture (Plate  II.,  Fig.  6). 

In  order  to  appreciate  more  fully  the  function  and  the 
morphological  relations  of  this  structure,  it  is  necessary  to 
glance  at  the  conditions  that  obtain  in  the  several  classes  of 
vertebrate  animals.  In  ova  that  develop  outside  of  the  body 
of  the  parent  organism,  a  special  dower  of  pabulum  is  pro- 
vided for  the  nutrition  of  the  embryo ;  this  dower  is  repre- 
sented by  the  deutoplasm  so  abundant  in  telolecithal  ova. 
In  the  case  of  amphibians,  whose  cleavage,  it  will  be 
remembered,  is  holoblastic  or  total,  the  cells  richest  in  deuto- 
plasm are  accumulated,  after  segmentation,  in  the  floor  of  the 
archenteron ;  this  accumulation  produces  on  the  future  ventral 
surface  of  the  embryo  a  marked  bulging,  which  constitutes 
the  amphibian  yolk-sac.  As  the  embryo  grows,  it  draws 
upon  this  store  for  its  nutrition,  in  consequence  of  which  the 
sac   gradually  shrinks,  its  cells   being,  for  the  most  part, 


THE   YOLK-SAC.  79 

liquefied  and  absorbed,  while  some  of  them  contribute  to  the 
lining  of  the  intestinal  canal. 

In  a  higher  type,  as  exemplified  in  sharks  and  dog-fishes, 
the  volk-sac  is  produced  by  a  folding-in  of  the  splanchno- 
pleure  and  the  somatopleure,  the  walls  of  the  sac  being 
therefore  constituted  by  both  of  these  layers ;  this  folding-in 
divides  the  archenteron  into  a  smaller  part,  the  intestinal 
canal,  lying  within  the  body  of  the  embryo,  and  a  larger 
cavity,  the  yolk-sac,  situated  outside  of  that  body.  The 
splanchnopleuric  layer  of  the  yolk-sac  is  continuous  with  the 
wall  of  the  intestinal  canal,  while  its  soraatopleuric  layer  is 
continuous  with  the  body-wall.  A  system  of  blood-vessels 
develops  upon  the  yolk-sac,  their  function  being  to  convey 
the  nutritive  material  into  the  body  of  the  embryo.  These 
blood-vessels  constitute  the  so-called  vascular  area,  which 
appears,  in  surface  views,  as  a  zone  encircling  the  embryonic 
area,  and,  later,  the  embryo,  since  the  latter  reposes  upon  the 
proportionately  much  larger  yolk-sac.  As  the  contents  of 
the  sac  become  absorbed,  the  latter  shrinks,  the  splanchno- 
pleuric layer  slipping  into  the  abdomen  of  the  embryo 
through  the  umbilical  opening,  the  somatopleuric  layer  con- 
tracting to  close  that  aperture. 

In  the  Amniota  (p.  75)  the  development  and  structure  of 
the  yolk-sac  are  modified  by  the  presence  of  the  amnion.  In 
these  groups  the  yolk-sac  and  the  gut  result  from  the  division 
of  the  archenteron  by  the  folding-in  of  the  splanchnopleure 
alone,  since  the  somatopleure  grows  away  from  the  splanchno- 
pleure to  form  the  amnion-fold,  and  thus  only  partially  in- 
vests the  yolk-sac  (Plate  IL,  Fig.  4). 

Since  the  yolk-sac  contains  the  store  of  food  destined  for 
the  nutrition  of  embryos  that  develop  outside  of  the  maternal 
body,  and  since  the  mammalian  embryo,  which  leads  an  intra- 
uterine existence,  is  endowed  with  a  relatively  small  quantity 
of  such  store,  the  yolk-sac  of  mammals  would  seem  to  indi- 
cate the  descent  of  the  latter  from  oviparous  ancestors. 
Further  and  stronger  evidence  of  such  descent  is  found 
in  the  fact  that  the  eggs  of  the  lowest  order  of  mammals, 
the  Monotremata,  comprising  the  echidna  and  the  ornitho- 
rhynchus,  are  "laid"  and  undergo  exira-uierme  development. 


80  TEXT-BOOK  OF  EMBRYOLOGY. 

In  the  human  embryo  the  umbiHcal  vesicle  is  found  par- 
tially constricted  off  from  the  intestinal  canal  by  the  end  of 
the  second  week ;  by  the  end  of  the  third  week  the  separa- 
tion of  the  two  cavities  has  advanced  to  such  an  extent  that 
the  vitelline  duct  is  present,  the  sac  attaining  its  maximum 
size  by  about  the  fourth  week. 

The  function  of  the  umbilical  vesicle,  as  above  intimated,  is 
to  serve  as  the  organ  of  nutrition  for  the  embryo  during  a 
certain  period.  The  manner  in  which  its  blood-vessels  de- 
velop will  be  considered  in  Chapter  X.  Their  growth  pre- 
cedes that  of  the  intra-embryonic  portions  of  the  vascular 
apparatus,  the  vascular  area  of  the  yolk-sac  being  the  seat 
of  the  earliest  blood-vessel  formation.  The  vessels  find 
their  way  into  the  body  of  the  embryo  along  the  vitelline 
duct,  and  consist  of  two  vitelline  artei'ies  and  two  vitelline 
veins. 

With  the  development  of  the  allantois  the  yolk-sac  retro- 
gresses, the  allantois  succeeding  it  as  the  organ  of  nutrition 
and  respiration.  By  the  end  of  the  sixth  week  the  sac  has 
shrunk  to  a  narrow  stalk,  which  is  surrounded  by  the  en- 
larged amnion,  and  which  terminates  in  a  knob ;  at  birth, 
the  knob  lies  near  the  placenta  (Plate  V.,  Fig.  2),  and  the 
atrophic  remnant  of  the  stalk  is  one  of  the  constituents  of 
the  umbilical  cord. 

THE   ALLANTOIS, 

The  allantois  is  an  embryonic  structure  which  is  found  in 
those  vertebrates  possessing  an  amnion.  Its  growth  is  cor- 
related with  the  retrogression  of  the  umbilical  vesicle,  which 
structure  it  supplants  as  the  organ  of  nutrition  and  respira- 
tion for  the  embryo. 

Appearing  at  first  as  a  little  evagination  or  out-pocketing 
of  the  ventral  wall  of  the  gut-tract,  the  allantois  finally  be- 
comes a  pedunculated  sac  lying  in  the  extra-embryonic  part 
of  the  coelom  (Plates  II.  and  III.),  its  stalk  leaving  the 
body-cavity  proper  through  the  umbilical  opening.  Being 
an  outgrowth  from  the  intestinal  canal,  the  walls  of  the 
allantois  are  made  up  of  splanchnopleure — that  is,  of  ento- 


PLATE  III. 


Vascular  villi  of 
placental  chorion. 


Body-cavity.   \ 

Vitelline  vesicle. 


Embryo. 


Space  bet-jjee7t  amnion 
and  chorion. 


Allantoic  blood-vessels. 


Allantoic  stalk. 


Diagrams  illustrating  the  later  stages  of  the  lormatiou  of  the  mammalian  fetal  mem- 
branes (modified  from  Roule). 


THE  ALLANTOIS. 


81 


derm  and  visceral  mesoderm.  Blood-vessels  develop  in  the 
mesodermic  stratum,  the  principal  trunks,  the  two  allan- 
toic arteries  and  veins,  being  connected  at  their  proximal 
ends  with  the  primitive  heart ;  this  system  of  vessels  consti- 
tutes the  allantoic  circulation  and  is  the  avenue  through 
which  the  growing  embryo  is  supplied  Avith  nutritive  mate- 
rial and  oxygen.  As  the  fundus  of  the  allantois  increases 
in  size,  it  spreads  itself  out  upon  the  inner  surface  of  the 
false  amnion  or  chorion  (Plate  III.,  Fig.  1),  into  whose  villi 
its  vascular  tissue  penetrates,  and  with  which  it  becomes 
intimately  blended.  The  union  of  the  allantois  and  the  false 
amnion  produces  the  true  chorion  of  some  authors. 

The  human  allantois  presents  a  striking  peculiarity  as  com- 
pared Avith  that  of  birds  and  reptiles ;  in  man,  the  allantois 


■'timmii 


Fig.  38.— Diagrammatic  sections  representing  growth  and  arrangement  of  the  am- 
nion in  the  earliest  stages  of  the  human  embryo  (His). 

develops  not  as  a  free  sac  projecting  into  the  extra-embryonic 
body-cavity,  but  as  a  mass  of  splanchnopleuric  tissue  which 
contains  only  a  rudimentary  cavity  and  which  grows  into' 
the  abdominal  stalk  (Fig.  38  and  Fig.  47,  hsi),  being  guided! 
by  that  structure  to  the  chorion. 

The  function  of  the  allantois  in  egg-laying  animals,  and 
possibly  in  some  others,  is  to  serve  as  a  nutritive  and  re- 
spiratory organ  and  as  a  receptacle  for  the  fetal  urine ;  in 


82  TEXT-BOOK  OF  EMBRYOLOGY. 

man  its  cavity  is  exceedingly  minute,  and  its  chief  function 
is  to  furnish  a  means  of  conveying  blood-vessels  from  the 
embryo  to  the  chorion. 

The  part  of  the  allantois  contained  within  the  body  of  the 
embryo  produces  three  structures  of  the  adult  organism  :  1, 
the  urachus,  an  atrophic  cord  extending  from  the  summit  of 
the  bladder  to  the  umbilicus;^  2,  the  urinary  bladder;  and  3, 
the  first  part  of  the  urethra  of  the  male,  or  the  entire  female 
urethra.  The  extra-embryonic  portion  shrinks  after  the 
appearance  of  the  placenta  and  forms  one  of  the  constituents 
of  the  umbilical  cord,  its  blood-vessels  becoming  the  umbil- 
ical arteries  and  veins. 

THE  CHORION. 

At  the  time  when  the  false  amnion  is  forming,  the  attenu- 
ated zona  pellucida  still  surrounds  the  embryonic  vesicle 
as  the  so-called  prochorion,  which  unites  with  the  false  am- 
nion, producing  the  primitive  chorion.  After  the  allantois 
has  grown  forth  from  the  gut-tract  and  has  spread  itself  over 
the  inner  surface  of  the  primitive  chorion,  it  becomes  blended 
with  the  latter  to  constitute  the  true  chorion  of  some  authors. 
The  chorion,  according  to  the  above  nomenclature,  may  be 
defined  as  the  membrane  which  encloses  the  germ  at  the 
stage  following  the  appearance  of  the  amnion  and  the  false 
amnion,  and  which  has  resulted  from  the  fusion  of  the  allan- 
tois with  the  primitive  chorion ;  or,  ignoring  the  zona  pellu- 
cida, the  chorion  results  from  the  fusion  of  the  allantois  and 
the  false  amnion.  Minot,  however,  defines  the  chorion  as  all 
that  part  of  the  extra-embryonic  somatopleure  wliich  is  not 
used  in  forming  the  true  amnion,  and  hereafter  in  this  work 
the  word  will  be  used  in  this  sense.  This  definition  limits 
the  term  to  the  outermost  covering  of  the  germ  after  the 
formation  of  the  amnion  (Plate  III.,  Fig.  1). 

The  chorion  consists  of  an  outer  ectodermic  layer  and 
an  inner  lamella  of  mesodermic  tissue.  The  mesoblastic  layer 
is  thin,  being  composed  of  from  two  to  four  layers  of  round, 

^  If  the  urachus  remains  patulous  instead  of  becoming  impervious,  urine 
may  escape  at  the  umbilicus,  and  the  condition  is  a  variety  oi  urinary  fistula. 


THE  CHORION.  83 

oval,  or  fusiform  cells,  and  is  at  first  devoid  of  blood-vessels. 
The  latter,  in  the  form  of  capillaries,  make  their  appearance 
at  some  time  during  the  second  week,  probably  as  extensions 
of  the  blood-vessels  of  the  allantois. 

The  outer  ectodermic  or  epiblastic  cells  of  the  chorion  at 
a  very  early  period,  certainly  as  early  as  the  third  day, 
undergo  proliferation  to  form  a  layer  of  tissue  called  the 
trophoblast,  which  is  from  one  to  several  layers  of  cells  in 
thickness.  The  trophoblast  layer  is  thickest  at  the  place  of 
attachment  of  the  ovum  to  the  uterine  mucosa.  The  inner 
cells  of  the  trophoblast  are  cubical,  and  have  large,  finely 
granular,  round  or  oval  nuclei.  In  the  youngest  human 
ovum  as  yet  examined — that  of  Peters,  estimated  to  be  three 
or  four  days  old — the  trophoblast  was  found  to  present  many 
little  jDrojections  in  the  form  of  strands  and  buds,  these 
being  the  foundations  of  the  future  villi  of  the  chorion.  The 
trophoblast  layer  was  not  solid,  but  was  honeycombed  with 
little  spaces  or  vacuoles  filled  with  maternal  blood,  which 
spaces  were  partly  lined  with  a  nucleated  protoplasm,  the 
early  syncytium  (Plate  IV.,  «).  Even  at  this  early  stage, 
therefore,  when  the  trophoblast  strands  or  early  villi  are  as 
yet  devoid  of  a  mesoblastic  element,  they  are  bathed  with 
the  maternal  blood.  Very  soon  the  mesoblastic  tissue  of 
the  chorion  grows  into  the  trophoblast  strands,  thus  forming 
the  permanent  villus  stems ;  and  during  the  second  week 
capillaries  extend  into  the  stems^  completing  the  formation 
of  the  fully  developed  vUli  of  the  chorion. 

The  early  development  of  villi  is  characteristic  of  the 
human  chorion  (Fig.  39  and  Plate  IL,  Fig.  6).  At  first  the 
villi,  either  covering  the  entire  surface  of  the  chorion  or 
leaving  the  two  opposite  poles  free,  are  of  uniform  size  ;  in 
the  latter  half  of  the  first  month,  however,  there  begins  to 
be  a  differentiation  into  a  region  containing  smaller,  and  one 
having  larger  and  more  numerous,  projections.  The  differ- 
ence between  the  tvA^o  areas  becoming  more  marked,  the 
relatively  smooth  part  of  the  membrane,  possessed  of  rudi- 
mentary villi,  is  designated  the  chorion  leve,  while  the 
region   with   well-developed    villous    projections    is    distin- 


84  TEXT-BOOK  OF  EMBRYOLOGY. 

guished  as  the  chorion  frondosum  (Plate  III.,  Figs.  1  and  2) ; 
the  latter  acquires  a  close  relation  with  the  macous  membrane 
of  the  uterus  and  becomes  the  fetal  part  of  the  placenta. 
The  villi  in  their  earlier  condition  are  somewhat  club-shaped 
elevations,  which  later  become  branched  to  form  secondary 
villi.  Each  fully  developed  villus  consists  of  a  core  of 
mesoblast,  covered  with  ectodermic  epithelium  and  contain- 
ing blood-vessels  (Fig.  44  and  Plate  lY.,  a).  Their  micro- 
scopic appearance  is  so  characteristic  that  they  afford  a 
means  of  positively  determining  whether  a  mass  discharged 


Fig.  39. — Human  ovum  of  about  twelve      Fig.  40. — Front  view  of  ovum  shown  in 
days  (Eeicliert),  side  view.  Fig.  39. 

In  both  figures  the  villi  are  limited  in  distribution,  leaving  the  poles  free. 

from  the  uterus  is  or  is  not  a  product  of  conception.  The 
further  alterations  in  the  villi  as  well  as  in  the  trophoblast 
in  general,  including  the  syncytium,  will  be  considered  in 
Chapter  VI. 

A  chorion  is  present,  as  a  rule,  in  those  animals  whose 
embryos  develop  within  the  uterus  ;  this  would  include  the 
entire  class  Mammalia,  with  the  exception  of  the  mono- 
tremes,  whose  eggs  undergo  extra-uterine  development,  and 
the  marsupials,  whose  embryos,  though  nourished  in  the 
womb,  never  acquire  villi  on  the  serosa,  nutriment  being 
absorbed  by  simple  contact  of  the  latter  with  the  uterine 
mucous  membrane.  The  Mammalia  are  therefore  divided 
into  the  Achoria,  comprising  the  monotrcmes  and  marsupials, 
and  the  Choriata,  including  all  other  mammals. 


CHAPTER    yi 


THE  DECIDU/E  AND  THE  EMBEDDING  OF  THE  OVUM. 
THE  PLACENTA.     THE  UMBILICAL  CORD. 

THE  DECIDU/E  AND  THE  EMBEDDING  OF  THE  OVUM. 

The  decidu3e  (deciduous  or  caducous  membranes)  are  the 
hypertrophied  naucosa  of  the  uterus  so  developed  as  to 
form  not  only  a  lining  for  the  uterine  cavity,  but  also  an 
envelope  enclosing  the  ovum,  and  a 
specially  altered  part  which  serves 
as  a  bond  of  connection  between 
the  ovum  and  the  womb. 

During  the  four  or  five  days 
preceding  menstruation,  the  so- 
called  constructive  stage  of  the 
menstrual  cycle,  the  mucous  mem- 
brane of  the  womb  becomes  much 
thickened  and  unusually  vascular, 
the  purpose  of  these  changes  being 
evidently  the  preparation  of  the 
uterus  for  the  reception  of  the 
ovum  in  the  event  of  impregna- 
tion. If  impregnation  has  not 
occurred,  the  thickened  mucosa, 
the  decidua  menstrualis,  is  in  great 
part  cast  off  as  a  part  of  the  men- 
strual discharge ;  if,  on  the  other 
hand,  conception  has  taken  place, 
the  mucous  membrane  undergoes 
still  greater  hypertrophy.  On  sec- 
tion, it  is  seen  to  consist  of  a  super- 
ficial compact  stratum  and  a  deeper 
spongy  layer  reposing  directly  upon  the  muscular  wall  of  the 
uterus.     In  the  compact  layer  are  the  necks  of  the  much 

85 


Jkfuscu/atZire 

Fig.  41. — Cross  section  through 
the  mucous  membrane  of  the 
uterus  at  the  beginning  of  preg- 
nancy (after  Kundrat  and  En- 
gelmann). 


86  TEXT-BOOK  OF  EMBRYOLOGY. 

enlarged  uterine  glands,  while  in  the  spongy  layer  are  their 
greatly  branched  and  often  tortuous  bodies  (Fig.  41).  The 
tortuosity  and  division  of  the  deeper  extremities  of  the  glands 
produce  the  characteristic  appearance  of  a  section  of  the 
spongy  stratum. 

The  alterations  necessary  to  convert  the  menstrual  decidua 
into  the  deciduae  of  pregnancy  take  place  in  part  while  the 
ovum  is  still  in  the  Fallopian  tube  ;  when  it  reaches  the 
uterus  it  becomes  attached  to  the  mucous  membrane  of  the 
latter,  usually  along  the  upper  part  of  the  posterior  wall.  A 
portion  of  the  mucous  membrane  eventually  comes  to  enclose 
the  ovum  as  in  a  distinct  envelope  (Plate  V.,  Fig.  1).  The 
part  of  the  uterine  mucosa  which  thus  surrounds  the  ovum 
is  the  decidua  reflexa ;  the  part  still  lining  the  cavity  of  tlie 
womb  is  the  decidua  vera ;  the  part  that  is  in  contact  with 
the  chorion  frondosum  is  the  decidua  serotina.  The  decidua 
serotina  afterward  becomes  the  maternal  part  of  the  placenta, 
intimately  uniting  with  the  chorion  frondosum. 

Until  recently  it  was  believed  that  the  ovum  became  im- 
planted upon  the  surface  of  the  mucosa,  and  that  the  latter 
grew  up  around  and  over  it  to  form  the  decidua  reflexa. 
This  theory  has  been  completely  set  aside  by  the  recent 
investigations  of  Peters  of  Vienna,  whose  results  have  been 
confirmed  by  Webster  of  Chicago.  Peters'  observations 
were  made  upon  the  gravid  uterus  of  a  suicide,  the  ovum 
being  embedded  in  a  triangular  prominence  on  the  upper 
median  region  of  the  posterior  uterine  wall.  The  ovum 
measured  in  three  diameters  respectively  1.6,  0.8,  and  0.9 
mm.,  its  estimated  age  being  three  or  four  days. 

The  embedding  of  the  ovum  (Plate  IV.),  or  its  sinking  into 
the  mucosa,  is  quickly  accomplished  by  the  erosion  of  the 
superficial  layers  of  the  latter,  presumably  by  the  phagocytic 
action  of  the  trophoblast.  Actual  erosion  is  evident  from 
the  absence  of  the  surface  epithelium  at  this  place.  The 
ovum  is  thus  brought  into  relation  with  the  deeper  layers  of 
the  mucosa.  The  edges  of  the  excavation  are  undermined, 
so  that  the  ovum  is  partly  covered  by  the  mucosa,  the  area 
not  thus  covered  being  occupied  by  an  organized  blood  clot, 


Plate  iv. 


o    a  ( 


°    n  i 

CD     ^  t^  O 

. .     3  C  <! 

S  '^  c 


iU    o'^riii:::jotD>. 


W     g,    (U     & 


p    o 


B-  P  p    " 

§  -5  '13.  -^ 

e.  B  1^  s 

X.  .  <^  •• 


tq-^^ 


THE  DECIDU^.  87 

the  tissue  fungus  (Plate  IV.)-  The  overhanging  edges  of  the 
excavation  constitute  the  beginning  of  the  reflexa,  which  is 
obviously,  therefore,  not  produced  by  the  upgrowth  of  a 
circular  fold  of  mucosa.  The  trophoblast  strands  or  early 
villi  extend  toward  and  into  the  serotina,  to  which  some  of 
them  become  attached.  It  is  thought  by  Webster  that  they 
may  absorb  fluid  and  nutriment,  and  that  by  phagocytic  action 
they  open  up  the  blood-spaces  of  the  serotina,  thus  bringing 
them  into  communication  with  the  lacunse  of  the  trophoblast. 

The  blood-lacunse  of  the  trophoblast  form  a  system  of  in- 
tercommunicating spaces,  the  beginnings  of  the  later  inter- 
villous spaces  of  the  placenta ;  they  are  filled  with  maternal 
blood  from  the  serotina,  and  are  lined  with  syncytium  (Fig. 
43),  the  latter  being  thin  in  places  and  resembling  an  endo- 
thelium. There  is  no  extension,  however,  of  the  endothelium 
of  the  serotinal  vessels,  either  upon  the  villi  or  into  the 
spaces  (Peters  and  Webster). 

The  syncytium  is  the  more  or  less  irregular  layer  of  nu- 
cleated protoplasm  which  appears  upon  the  surface  of  the 
ovum  toward  the  end  of  the  first  week,  lining  the  tropho- 
blast lacunse,  and  later  penetrating  as  irregular  masses  into 
the  serotina,  where  it  is  found  until  the  end  of  pregnancy. 
No  traces  of  it  are  found  on  the  vera  after  the  sixth  week. 
The  origin  of  the  syncytium  has  long  been  in  dispute.  Peters 
has  shown  that  it  results  from  the  transformation  of  the 
superficial  part  of  the  trophoblast,  probably  from  contact  of 
the  latter  with  maternal  blood,  which,  he  thinks,  exercises  a 
blending  influence  upon  the  trophoblast  cells,  so  that  as  indi- 
vidual cells  they  disappear,  the  result  being  a  non-cellular 
but  nucleated  protoplasm.  Peters  also  believes  that  cor- 
puscles of  the  maternal  blood  are  appropriated  by  the 
syncytium,  and  that  the  latter,  covering  villi  and  chorion  as 
it  does,  has  something  to  do  with  the  interchange  of  nutri- 
ment and  waste  products  between  the  maternal  blood  and 
the  ovum.  What  remains  of  the  early  trophoblast  after  the 
formation  of  the  syncytium  is  the  layer  of  Langhans. 

The  ciliated  epithelium  of  the  uterine  mucous  membrane 
disappears  by  the  end  of  the  first  month  of  pregnancy  (Minot) ; 


88  TEXT-BOOK  OF  EMBRYOLOGY. 

somewhat  later,  that  of  the  uterine  glands  is  also  lost.  By 
the  end  of  the  fifth  month  the  fetus  and  its  appendages  have 
increased  in  size  to  such  an  extent  that  they  completely  fill 
the  cavity  of  the  womb,  and  the  space  between  the  vera  and 
the  reflexa  is  obliterated.  After  the  second  month  the  vera 
becomes  progressively  thinner  and  the  reflexa  undergoes 
degenerative  changes  to  such  a  degree  that  by  the  end  of 
pregnancy  merely  remnants  of  it  are  present.^  By  the  sixth 
month  the  vera  is  intimately  blended  with  the  chorion. 

THE  PLACENTA. 

The  placenta,  in  certain  groups  of  mammals,  including 
man,  is  the  organ  of  nutrition"  for  the  fetus  during  about  the 
latter  two-thirds  of  the  period  of  gestation.  In  man,  it  is 
a  discoid  structure,  attached  by  one  surface  to  the  wall  of 
the  womb,  and  connected  on  its  opposite  aspect  with  the 
fetus  through  the  medium  of  the  umbilical  cord. 

The  human  placenta  represents  the  highest  specialization 
of  an  apparatus  for  bringing  the  fetal  blood  into  intimate 
relation  with  the  blood  of  the  mother.  In  eggs  that  develop 
outside  of  the  body  of  the  mother,  such  as  those  of  reptiles, 
birds,  and  the  lowest  mammals,  the  Monotremata,  the 
growing  embryo  necessarily  acquires  no  connection  with 
the  uterine  mucosa,  but  draws  upon  its  original  dower  of 
nutriment,  the  deutoplasm,  until  its  development  is  com- 
pleted, when  it  breaks  through  the  shell  and  seeks  its  own 
food ;  in  these  groups  the  chorion  develops  no  villi.  In 
the  marsupials,  a  group  of  mammals  higher  than  the  mono- 
tremes,  the  ovum,  although  developing  in  the  uterus,  forms 
no  close  connection  with  it,  but  obtains  its  nourishment  by 
simple  imbibition  from  the  uterine  mucosa.  On  the  other 
hand,  in  all  mammals  higher  than  monotremes  and  marsu- 
pials, the  chorion  is  distinguished  by  the  presence  of  villi 
upon  its  surface. 

Between  the  human  placenta  and  the  non-villous  chorion 
of  the  marsupials,  certain  gradations  exist;  for  example,  in 
pigs,  whales,  and  some  others,  there  is  no  proper  placenta, 

'  Webster. 


THE  PLACENTA. 


89 


the  villi  being  evenly  distributed  over  the  surface  of  the 
chorion ;  while  in  ruminants  the  villi  are  grouped  into  little 
clusters  or  tufts  called  cotyledons,  which  are  easily  detachable 
from  the  mucous  lining  of  the  womb.  Owing  to  this  loose 
connection,  the  uterine  mucosa  is  non-deciduous — that  is,  it  is 
not  cast  off  after  the  birth  of  the  young.  The  foregoing 
classes  are  therefore  styled  Mammalia  indeciduata,  in  contra- 
distinction to  the  Mammalia  deciduata — including  man, 
rodents,  and  Insectivora — in  which  there  is  a  loss  of  the 
greater  part  of  the    uterine    mucous    membrane    after    the 


Pig.  42. — Mature  placenta:  a,  entire  organ,  showing  fetal  surface  with  membranes 
attached  to  the  periphery;  6,  a  portion  of  attached  surface. 

expulsion  of  the  fetus.  In  the  Carnivora  the  placenta 
has  the  form  of  a  zone  or  ring — placenta  zonaria — while  in 
man  and  certain  allied  mammals,  as  apes,  rodents,  and  some 
others,  it  is  discoid  in  shape — placenta  discoidea. 

The  human  placenta  is  formed  in  the  third  month  of  preg- 
nancy ;  since  it  results  from  the  union  of  the  chorion  frondo- 
sum  with  the  decidua  serotina,  it  consists  of  a  fetal  and  a 
maternal  part. 

Our  conceptions  of  the  development  of  the  placenta  must 
be  modified  to  accord  with  recent  investigations.     The  em- 


90 


TEXT-BOOK  OF  EMBRYOLOGY. 


bedding  of  the  ovum  in  the  uterine  mucosa  and  the  modifica- 
tions occurring  in  the  chorion,  including  the  growth  of  its 
villi  and  its  differentiation  into  the  chorion  frondosum  and 
the  chorion  leve,  have  been  considered  above.  It  will  be 
recalled  that  the  ovum  eats  its  way,  as  it  were,  into  the 
mucosa,  thus  causing  the  superficial  layers  of  the  latter  to 
disappear  at  the  site  of  implantation.  It  is  possibly  this 
process  of  erosion  upon  the  part  of  the  fetal  trophoblast 
that  opens  up  the  already  dilated  capillaries  of  the  serotina — 
the  sinuses — and  alloAvs  the  maternal  blood  access  to  the 
blood-lacunffi  of  the  trophoblast,  where  it  thus  bathes  the 
primitive  villi.     Since  the  entire  trophoblast  is  vacuolated, 


Fig.  43.— Schematic  representation  of  the  development  of  the  placenta  (after 
Peters):  M.,  mesoblast;  Tr.,  trophoblast;  Bl.  L.,  blood  lacunse;  Sy.,  syncytium; 
En.,  endothelium;  Ca.,  maternal  capillaries;  LZ.,  circumvallation  zone;  S])., 
spongiosa  of  serotina. 

the  maternal  blood  at  this  time — the  first  week — is  brought 
into  relation  with  the  whole  surface  of  the  chorion.  In  the 
latter  half  of  the  first  month  the  distinction  betAveen  chorion 
frondosum  and  chorion  leve  begins  to  be  manifest,  the  villi 
of  the  latter  gradually  retrograding  until,  in  the  sixth  week, 
they  are  greatly  degenerated,  many  of  them  being  without 
blood-vessels. 

The  villi  of  the  chorion  frondosum  increase  in  size,  number, 
and  complexity,  some  of  them  acquiring  attachment  to  the 
serotina,  but  not,  as  a  rule,  entering  the  serotinal  blood  sinuses. 
The  formation  of  new  villi  continues  throughout  pregnancy. 
The  villi  acquire  capillaries  in  the  second  week,  these  being 


THE  PLACENTA. 


91 


extensions  of  the  allantoic  blood-vessels.  The  syncytium  of 
the  chorionic  lacunae,  now  the  intervillous  spaces,  increases  in 
quantity,  and  not  only  lines  the  spaces,  but  exists  in  the  form 
of  masses,  some  of  which  become  attached  to  the  serotina  be- 
tween the  villi,  while  others  penetrate  into  it,  many  being  found 
at  the  fourth  month  in  the  serotinal  connective-tissue  spaces. 
The  decidua  serotina  (basal  decidua)  in  the  first  month  is 
edematous  and  hyperemic,  presenting  dilated  capillaries  and 
blood-spaces,  many  of  which  communicate  with  the  inter- 
villous spaces  of  the  chorion.  By  the  sixth  week  its  surface 
epithelium  is  entirely  lost,  and  the  parts  of  its  glands  con- 
tained in  the  compacta  are  to  a  great  extent  obliterated.     In 


Fig.  44.— Schematic  representation  of  the  development  of  the  placenta  (after 
Peters):  M.,  mesoblast;  Tr.,  trophoblast;  Sy.,  syncytium;  En.,  endothelium;  Ca.. 
maternal  capillaries ;  /.c,  fetal  capillaries;  d.s.,  decidual  septum;  F.b.,  fibrin; 
i.R.,  intervillous  space. 


the  fourth  month  it  is  thinner,  more  irregular  in  thickness, 
contains  less  sinuses,  and  shows  degeneration  in  the  com- 
pacta, with  many  masses  of  syncytium.  Toward  the  end 
of  pregnancy  the  sinuses  increase  in  size,  and  the  irregu- 
larity in  thickness  and  the  degeneration  are  more  marked. 
The  placenta  at  term  is  a  discoid  mass,  in  situ,  but  less 
flattened  after  its  expulsion  from  the  uterus.  Its  diameter 
is  from  15  to  20  and  its  thickness  from  3  to  4  centimeters. 
The  uterine  surface  is  convex  and  irregular,  and  is  im- 
perfectly divided  into  tufts  or  cotyledons.  The  somewhat 
concave  fetal  surface,  rather  mottled,  is  covered  by  the 
loosely  adherent  amnion,  and  presents,  usually  near  its  center, 


92  TEXT-BOOK  OF  EMBRYOLOGY. 

the  attachment  of  the  umbilical  cord.  The  maternal  part 
of  the  placenta,  the  decidua  serotina,  is  of  varying  thickness, 
in  some  places  being  absent.  Its  compact  layer  shows 
fibrinous  degeneration,  very  few  traces  of  glands,  and  no 
epithelium.  The  blood-spaces,  lined  with  endothelium  and 
representing  greatly  dilated  capillaries,  are  in  communication 
with  the  intervillous  spaces  of  the  fetal  placenta  (Plate  IV.,  a). 
Scattered  throughout  the  serotina  are  masses  of  syncytium. 
In  the  shed  placenta  there  is  very  little  of  the  serotina,  since 
separation  takes  jjlace  through  the  cornj^acta,  the  spongiosa 
and  a  part  of  the  compacta  remaining  upon  the  uterine  wall. 

The  fetal  part  comprises  almost  the  entire  thickness  of 
the  cast-off  placenta.  It  is  made  up  of  villi  ^  of  all  ages  and 
sizes  springing  from  the  chorion  (some  attached  distally  to 
the  serotina,  others  projecting  free  into  the  intervillous 
spaces),  and  of  masses  of  syncytium  attached  to  both  villi 
and  chorion  (Plate  IV.,  a).  The  intervillous  spaces  are  a 
system  of  intercommunicating  cavities  through  which  the 
maternal  blood  circulates  and  which  are  in  communication 
with  the  blood-spaces  of  the  serotina.  Elevations  of  the 
serotina  between  the  villi  constitute  the  septa  placentae. 
The  so-called  marginal  sinus  at  the  periphery  of  the 
placenta  is  merely  a  system  of  intervillous  spaces  that 
intercommunicate  more  freely  because  of  the  relative 
paucity  and  small  size  of  the  villi  in  this  region. 

The  site  of  attachment  of  the  placenta  to  the  uterus  is 
usually  the  upper  part  of  the  posterior  wall.  Under  certain 
circumstances  it  may  become  attached  lower  down,  even 
extending  partly  or  wholly  over  the  mouth  of  the  womb, 
constituting  then  the  condition  known  as  placenta  praevia. 

THE  UMBILICAL  CORD. 

The  blood-vessels  through  which  the  fetal  blood  finds  its 
way  from 'the  fetus  to  the  placenta  and  back  again  to  the 
fetus,  together  with  the  atrophic  vestiges  of  certain  structures 
associated  with  the  development  of  these  vessels,  constitute 
the  structure  known  as  the  umbilical  cord.  In  considering 
the  growth  of  the  human  allantois  it  was  pointed  out  that  the 
^  For  structure  of  villi,  see  pages  83,  84. 


Plate  iv  a. 


^v       \        ©  * 


Diagrammatic  representation  of  relationship  of  ovum  to  decidua:  1,  in  latter 
half  of  first  week ;  2,  a  few  days  later;  3,  a  few  months  later,  when  placenta  is  well 
defined  (Webster) :  n,  fetal  mesoblast,  showing  indications  of  beginning  extension 
into  trophoblast  stalks  m  1,  actual  extension  in  2  and  3:  b,  trophoblast,  being 
reduced  in  3  and  .constituting  here  the  layer  of  Langhans;  c,  trophoblast  lacuna 
in  1,  enlarged  in  2  and  3  as  an  intervillous  space ;  d,  syncytium,  seen  in  its  earliest 
stage  in  1 ;  e,  decidua ;  /,  maternal  blood-sinus ;  g,  endothelium  lining  maternal 
sinus;  h,  epiblastic  covering  of  cord;  i,  amniotic  epiblast ;  j,  umbilical  vein;  k, 
umbilical  artery ;  I,  amniotic  mesoblast ;  m,  extension  of  decidua  on  under  surface 
of  chorion  at  edge  of  placenta ;  w,  large  villus-stem. 


THE  PLACENTA.  93 

latter  structure,  as  it  grows  from  the  ventral  wall  of  the  gut- 
tract  into  the  so-called  allantoic  or  abdominal  stalk,  becomes 
the  seat  of  development  of  the  two  allantoic  arteries  and  of 
an  equal  number  of  allantoic  veins.  With  the  metamorphosis 
of  the  chorion  frondosum  into  the  fetal  placenta,  the  abdom- 
inal stalk  becomes  more  slender  and  at  the  same  time  much 
elongated,  and  the  allantoic  blood-vessels  are  henceforth  the 
umbilical  vessels.  The  two  umbilical  veins  fuse,  so  that,  at 
birth  and  for  some  time  before,  there  is  but  one  vein,  though 
there  are  still  two  arteries.  The  umbilical  vein,  entering  the 
body  of  the  fetus  through  the  umbilicus,  passes  directly  to 
the  under  surface  of  the  liver,  where  it  unites  with  the  fetal 
portal  vein  and  gives  off  a  branch  of  communication,  the 
ductus  venosus,  to  the  inferior  vena  cava,  after  which  it 
enters  the  liver  through  the  transverse  fissure.  The  umbilical 
arteries,  whose  intra-embryonic  portions  are  called  the  hypo- 
gastric arteries,  are  the  direct  continuations  of  the  superior 
vesical  arteries  of  adult  anatomy.  They  leave  the  body  of 
the  fetus  at  the  umbilicus. 

The  umbilical  cord,  while  consisting  essentially  of  the 
three  blood-vessels  mentioned,,  contains  also  the  remnant  of 
the  allantoic  stalk  and  of  the  umbilical  vesicle,  these  struct- 
ures being  surrounded  and  held  together  by  a  quantity  of 
embryonic  connective  tissue,  the  jelly  of  Wharton,  which 
makes  up  the  chief  part  of  the  mass  of  the  cord ;  upon  the 
surface  is  a  layer  of  epithelium,  continuous,  at  the  distal  end 
of  the  cord,  with  the  epithelium  of  the  amnion. 

The  umbilical  cord  has  an  average  length  of  55  cm.,  or  22 
inches,  but  varies  between  the  extremes  of  15  cm.  (6  inches) 
and  160  cm.  (64  inches);  its  thickness  is  about  1.5  cm. 
(|-  inch).  The  cord  presents  the  appearance  of  being  spi- 
rally twisted ;  it  is  probable,  however,  that  the  appearance 
of  torsion  is  conferred  by  the  spiral  or  coiled  arrangement 
of  its  arteries,  due  to  their  excessive  growth,  rather  than  by 
a  twist  of  its  entire  mass.  There  may  be  one  or  more  true 
knots  in  the  cord,  produced  by  the  slipping  of  the  fetus 
through  a  loop. 

The  position  of  attachment  of  the  cord  to  the  placenta  is 


94  TEXT-BOOK  OF  EMBRYOLOGY. 

usually  near,  but  seldom  exactly  in,  the  center  of  the  fetal 
surface  of  that  organ ;  rarely  it  may  be  found  attached  to 
its  edge,  and  still  more  rarely  to  the  fetal  membranes  them- 
selves at  some  little  distance  from  the  edge  of  the  placenta, 
with  which,  in  the  latter  case,  it  is  connected  by  its  blood- 
vessels. 

The  great  length  of  the  human  umbilical  cord  is  thought 
to  be  due  to  the  relatively  large  quantity  of  amniotic  fluid 
present  in  the  human  subject. 

After  birth,  the  portions  of  the  hypogastric  arteries  extend- 
ing from  the  upper  part  of  the  lateral  wall  of  the  bladder  to 
the  umbilicus  undergo  atrophy,  becoming  impervious  fibrous 
cords ;  the  intra-abdominal  part  of  the  umbilical  vein  like- 
wise becomes  atrophic  and  impervious,  constituting  the 
so-called  round  ligament  of  the  liver. 

RELATIONS  OF  THE   FETAL  MEMBRANES  AT  BIRTH. 

When  the  amniotic  fluid  attains  its  maximum  bulk — at 
about  the  end  of  the  sixth  month — it  requires  so  much  space 
that  it  presses  the  amniotic  membrane  closely  against  the 
chorion,  which  latter,  covered  by  the  remnants  of  the  re- 
flexa,  is  in  turn  forced  into  intimate  relation  with  the  vera 
(Plate  v.).  At  term  the  vera  and  chorion  have  become 
practically  one  membrane.  The  amnion,  while  adhering  to 
the  Inner  surface  of  the  chorion,  is  so  loosely  associated  with 
the  latter  that  it  may  be  peeled  off  from  it.  The  mem- 
branes, which  constitute  a  fluid-filled  sac  surrounding  the 
fetus,  are  ruptured  by  the  contractions  of  the  uterus  at  some 
time  during  parturition.  Through  this  rent  the  child  is 
forced  during  birth,  the  placenta  and  the  membranes  re- 
maining behind.  After  the  expulsion  of  the  child,  the  vera 
and  the  placenta  detach  themselves  from  the  uterine  wall, 
and,  with  the  chorion  and  the  amnion,  constitute  the  after- 
birth, which  is  expelled  shortly  after  the  expulsion  of  the 
child.  The  separation  of  the  after-birth  takes  place  in  the 
compact  layer  respectively  of  the  decidua  vera  and  of  the 
uterine  placenta.  The  spongy  layer  and  what  remains  of  the 
compacta  serve  for  the  regeneration  of  the  uterine  mucosa. 


Plate  V. 


CHAPTER  VII. 

THE  FURTHER  DEVELOPMENT  OF  THE  EXTERNAL 
FORM  OF  THE  BODY. 

Having  traced  the  growth  of  the  germ  to  the  time  when 
the  body  of  the  embryo  becomes  definitely  differentiated  from 
the  embryonic  appendages  or  fetal  membranes,  the  develop- 
ment of  the  individnal  organs  and  tissues  may  be  taken  up. 
The  discussion  of  this  latter  subject^  especially  of  that  part 
of  it  pertaining  to  the  structures  on  the  exterior  of  the  body, 
involves  a  consideration  of  the  external  form  of  the  embryo 
and  fetus  during  the  successive  stages  of  growth. 

In  the  preceding  chapters  it  was  pointed  out  that  the  cells 
of  the  segmented  ovum  arranged  themselves  in  such  a  man- 
ner as  to  form  a  hollow  sphere,  the  "blastodermic  vesicle 
(Plate  I.) ;  that  this  vesicle,  having  at  first  a  single-layered 
wall,  came  to  consist  of  two  layers  of  cells,  the  ectoderm  and 
the  entoderm;  and  that,  finally,  a  third,  intervening  layer, 
the  mesoderm,  made  its  appearance.  It  was  shown,  further, 
that  the  thickened  portion  of  the  vesicle  wall,  the  embryonic 
area,  became  more  and  more  differentiated  from  the  remain- 
der, and  that,  by  certain  processes  of  folding,  this  area  was 
made  to  assume  the  definite  form  of  the  embryonic  body, 
while  from  the  other  parts  of  the  vesicle-walls  the  fetal  mem- 
branes were  produced  (Plate  II.).  It  may  be  well  to  remind 
the  reader  again  that  when  the  body  of  the  embryo  has  be- 
come closed  off  from  the  fetal  membranes,  this  body  is  an 
irregularly  tubular  structure  whose  walls  are  the  somato- 
pleure  and  whose  enclosed  space  is  the  body-cavity,  and  that 
within  it  are  two  other  tubes,  a  larger,  the  gut-tract,  formed 
by  the  splanchnopleure,  and  a  smaller  ectodermic  tube,  the 
neural  canal. 

While,  as   a   matter   of  convenience,  the  description    of 

95 


96  TEXT-BOOK  OF  EMBRYOLOGY. 

the  individual  organs  is  taken  up  after  tracing  the  course 
of  development  to  this  stage,  it  should  be  borne  in  mind 
that  the  rudiments  of  some  of  them  are  already  distin- 
guishable before  the  germ-layers  become  infolded  to  form 
the  body-wall  and  the  gut-tract.  It  will  facilitate  a  compre- 
hension of  the  general  principles  concerned  in  the  origin  of 
the  different  parts  of  the  body  to  refer  to  the  tabulated  state- 
ment of  the  derivatives  of  the  three  primary  germ-layers  as 
presented  in  Chapter  III. 

In  considering  the  external  form  of  the  product  of  concep- 
tion, one  may  adopt  the  classification  of  His,  referred  to  in 
the  first  chapter.  This  author  divides  the  period  of  devel- 
opment into  three  stages,  of  which  the  first,  the  stage  of  the 
ovum,  or  the  blastodermic  stage,  comjjrises  the  first  and  second 
Aveeks  of  intra-uterine  growth  ;  the  second,  the  stage  of  the 
embryo,  extends  from  the  second  to  the  fifth  week  ;  and  the 
third,  or  the  fetal  stage,  includes  the  time  between  the  fifth 
week  and  the  end  of  gestation. 

THE  STAGE  OF  THE  OVUM. 

During  the  fortnight  allotted  to  this  first  stage  of  develop- 
ment occur  the  various  changes  by  which  the  impregnated 
ovum  acquires  the  form  of  a  hollow  sphere,  designated  the 
embryonic  or  blastodermic  vesicle.  The  series  of  transforma- 
tions has  been  described  in  Chapter  II,  In  this  place  it  will 
be  sufficient  to  refer  to  the  external  characters  of  the  blasto- 
dermic vesicle  as  depicted  in  Figs.  39,  40,  which  represent 
the  ovum  described  by  Reichert.  This  ovum  was  estimated 
to  be  about  twelve  days  old,  and  was  one  of  the  youngest 
human  ova  ever  described.  Its  form  was  that  of  a  sphere 
somewhat  flattened,  its  short  and  long  diameters  measuring 
respectively  3.3  mm.  and  5.5  mm.  The  flattened  surfaces 
were  smooth,  while  the  equatorial  zone  was  beset  with  villi. 
The  early  appearance  of  villi  is  characteristic  of  the  human 
ovum. 

THE  STAGE  OF  THE   EMBRYO. 

It  is  during  the  early  part  of  the  second  stage,  at  about 
the  fourteenth  day,  that  the  somatopleuric  layer  of  the  blasto- 


THE  STAGE  OF  THE  EMBRYO. 


97 


dermic  vesicle  becomes  folded  in  to  produce  the  walls  of  the 
embryonic  body.  Fig.  46  shows  a  human  embryo  of  about 
the  fifteenth  day,  whose  form  is  as  yet  imperfectly  differ- 
entiated, the  ventral  wall  of  the  body  being  incomplete, 
since  the  gut-tract  is  still  in  communication  with  the  umbili- 
cal vesicle  throughout  almost  the  entire  length  of  the  embryo. 
The  back  and  sides  of  the  embryo  are  enveloped  by  the 
amnion,  and  the  dorsal  outline  is  concave.     The  caudal  pole 


Fig.  45.— Human  embryo  of  about  the  thirteenth  day  (His).  The  caudal  pole  of 
the  embryo  is  connected  with  the  blastodermic  vesicle  by  means  of  the  abdominal 
or  allantoic  stalk ;  the  amnion  already  completely  encloses  the  embryo,  and  the 
large  vitelline  sac  communicates  throughout  the  greater  part  of  the  mitral  surface 
by  means  of  the  unclosed  gut-tract. 

is  seen  to  be  connected  by  means  of  the  allantoic  stalk  with 
the  primitive  chorion,  which  latter  structure,  however,  is  not 
represented  in  the  figure.  The  concavity  of  the  dorsal  out- 
line is  peculiar  to  the  human  embryo  of  this  stage.  The 
development  of  the  head  is  closely  associated  with  the  dilata- 
tion of  the  cephalic  end  of  the  neural  tube  and  the  subse- 
quent division  of  this  dilated  extremity  into  the  three  primary 
brain-vesicles,  the  fore-brain,  the  mid-brain,  and  the  hind- 
brain.  The  oral  pit,  the  first  indication  of  the  future  mouth, 
is  jof  esent  in  the  early  part  of  this  stage ;  it  is  a  depression 


98 


TEXT-BOOK  OF  EMBRYOLOGY. 


between  the  prominent  fore-brain  vesicle  and  the  cardiac 
dilatation  (Fig.  47). 


Fig.  46.— Human  embryo  of  about  the  fifteenth  day  (His).  The  embryo  is 
attached  to  the  wall  of  the  blastodermic  vesicle  by  means  of  the  umbilical  or 
allantoic  stalk,  and  is  enclosed  within  the  amnion  ;  the  large  vitelline  sac  freely 
communicates  with  the  still  widely  open  gut. 


Fig.  47. — Human  embryo  with  yolk-sac,  amnion,  and  belly-stalk  of  fifteen  to 
eighteen  days  (after  Costc).  The  chorion  is  detached  at  am' :  am,  amnion ;  am',  the 
point  of  attachment  of  the  amnion  to  the  chorion  drawn  out  to  a  tip ;  hst,  belly- 
stalk  ;  Sch,  tail-end ;  us,  primitive  segment ;  dg,  vitelline  blood-vessels ;  ds,  yolk- 
sac;  h,  heart;  vb,  visceral  arch. 


Between  the  fifteenth  and  the  twenty-first  day,  the  lens- 
vesicles  and  the  otic  vesicles  are  formed  by  invaginations  of 
the  surface  ectoderm  (Fig.  48,  7  and  8),  these  sacs  being 


THE  STAGE  OF  THE  EMBRYO. 


99 


Fig.  48.— Early  human  embryos,  all  enlarged  about  two  and  a  half  times  (His)  : 
1-4,  from  twelfth  to  fifteenth  day ;  5,  6,  from  eighteenth  to  twenty-first  day ;  7,  8, 
from  twenty-third  to  twenty-fifth  day ;  9-12,  from  twenty-seventh  to  thirtieth  day  ; 
13-17,  from  thirty-first  to  thirty-fourth  day.  am,  amnion ;  uv,  umbilical  or  vitelline 
vesicle ;  als,  allantoic  or  abdominal  stalk ;  c,  c',  brain-vesicles ;  h,  heart ;  va,  visceral 
arches ;  o,  optic  vesicle ;  ot,  otic  vesicle ;  ol,  olfactory  pit ;  I,  I',  upper  and  lower 
extremities  ;  s,  somites  ;  cd,  caudal  process ;  u,  primitive  umbilical  cord. 


100  TEXT-BOOK  OF  EMBRYOLOGY. 

the  radiments  respectively  of  the  crystalline  lens  and  of  the 
membranous  internal  ear ;  at  this  time  also  the  visceral  arches 
and  clefts  first  become  distinguishable.  On  the  twenty-first 
day,  the  rudiments  of  the  limbs  appear  as  little  bud-like 
processes  springing  from  the  trunk.  The  conspicuous  pro- 
jection on  the  ventral  surface  between  the  now  almost  com- 
pleted yolk-sac  and  the  cephalic  end  of  the  body  is  produced 
by  the  primitive  heart  (Fig.  48,  10,  11,  and  12). 

Until  the  twenty-firvSt  day  the  embryonic  body  is  erect. 
Between  the  twenty-first  and  twenty-third  days  a  marked 
alteration  in  the  appearance  of  the  germ  is  brought  about  by 
a  pronounced  bending  of  the  long  axis  of  the  embryonic 
body  (Fig.  48).  The  degree  of  curvature  is  such  that  the 
caudal  and  cephalic  extremities  overlap.  The  flexion  reaches 
its  maximum  degree  by  the  twenty-third  day.  The  curved 
dorsal  outline  is  referable  to  four  well-marked  flexions,  the 
position  of  the  most  anterior,  or  cephalic  flexure,  correspond- 
ing to  that  of  the  future  sella  turcica  and  being  indicated  by 
the  projection  of  the  mid-brain  vesicle  (Fig.  51,  III.) ;  at  this 
point  the  anterior  part  of  the  head  is  bent  almost  sufficiently 
to  form  a  right  angle  with  the  posterior  half.  A  second  or 
cervical  flexure  is  found  in  the  future  neck-region,  while 
further  caudad  are  seen  the  less  pronounced  dorsal  and  coccy- 
geal curves. 

The  fourth  week  marks  the  period  of  the  most  active 
growth  of  the  embryo.  After  the  twenty-third  day,  the 
body  as  a  whole  uncoils  somewhat,  although  in  the  latter 
half  of  the  fourth  week  the  individual  flexures  noted  above 
become  more  conspicuous. 

The  Visceral  Arches  and  Clefts. — The  visceral  arches, 
with  the  intervening  visceral  clefts,  constitute  a  conspicuous 
feature  of  the  external  appearance  of  the  embryo  during  this 
stage.  These  arches  are  a  series  of  five  approximately 
parallel  ridges  appearing  upon  each  side  of  the  future 
neck-region  and  extending  obliquely  downward  and  for- 
ward toward  the  ventral  surface  of  the  embryo  (Figs.  49 
and  51).  The  four  furrows  lying  between  the  five  visceral 
arches  are  the  visceral  clefts.     A  coronal  section  of  tho  neck- 


Fig.  49.— Human  embryo  of  about  three  weeks,  showing  visceral  arches  and  fur- 
rows and  their  relations  to  aortic  arches  (His) :  mx,  mn,  maxillary  and  mandibular 
processes  of  first  visceral  arch  ;  a  I-a  IV,  first  to  fourth  aortic  arches  ;  jv,  cv,  primi- 
tive jugular  and  cardinal  veins ;  dC,  duct  of  Cuvier;  at,  v,  atrium  and  ventricle  of 
primitive  heart ;  vs,  vitelline  sac ;  va,  da,  ventral  and  dorsal  aortse ;  ov,  ot,  optic  and 
otic  vesicles ;  uv,  ua,  umbilical  veins  and  arteries  ;  vv,  vitelline  vein ;  al,  allantois. 

101 


102 


TEXT-BOOK  OF  EMBRYOLOGY. 


region  (Fig.  50) — a  section  in  a  plane  parallel  with  the  ventral 
surface — shows  that  the  furrows  seen  on  the  ectoclermic 
surface  correspond  in  position  to  a  like  number  of  deeper 
grooves  on  the  inner  or  entodermic  surface.  The  inner 
furrows  are  out-pocketings  of  the  entoderm  lining  the  phar- 
yngeal region  of  the  fore-gut;  they  are  referred  to  as  the 
pharyngyeal  pouclies  or  throat-pockets  to  distinguish  them  from 
the  outer  clefts.  At  the  bottom  of  the  clefts  the  ectoderm  is 
in  contact  with  the  entoderm,  the  mesoderm  being  absent ; 
these  two  layers  constitute  the  closing  membrane.  The  vis- 
ceral arches  or  ridges  consist  of  thickened  masses  of  meso- 
dermic  tissue  covered  outwardly  and  inwardly  respectively 


^ 


B 

Fig.  50.— Coronal  sections  of  two  human  embryos,  showing  ventral  wall  of 
pharyngeal  end  of  gut-tract  from  behind  (from  Tourneux,  after  His).  A,  from 
embryo  of  3.2  mm. ;  B,  of  4.25  mm.  (about  25  to  30  days).  /,  II,  III,  IV,  outer  vis- 
ceral furrows  ;  V,  sinus  preecervicalis,  comprising  third  and  fourth  outer  furrows ; 
1, 2,  3,  U,  visceral  arches,  each  with  its  visceral-arch  vessel ;  G,  tuberculum  impar ; 
7,  orifice  of  larynx  ;  S,  pulmonary  evagination. 


by  the  ectoderm  and  the  entoderm.  Each  arch  contains  an 
artery,  the  visceral-arch  vessel.  These  five  pairs  of  visceral- 
arch  vessels  arise  by  a  common  stem,  the  truncus  arteriosus, 
from  the  primitive  heart.^ 

The  morphological  significance  of  the  visceral  arches  and 
clefts  may  be  appreciated  by  a  comparison  of  the  conditions 
obtaining  in  lower  types.     While  in  birds  and  mammals  the 

^  For  an  account  of  the  metamorphosis  of  the  visceral-arch  vessels  into 
the  adult  arteries  of  the  throat  and  neck  the  reader  is  referred  to  Chapter  X. 


THE  STAGE  OF  THE  EMBRYO.  103 

number  of  the  clefts  is  four,  in  reptiles,  amphibians,  and 
bony  fishes,  five  clefts  appear,  and  in  some  fishes  (selachians) 
the  number  is  six.  In  all  aquatic  vertebrates,  the  thin 
epithelial  closing  membranes  rupture,  thus  establishing  com- 
munications between  the  alimentary  tract  and  the  exterior, 
through  which  openings  water  passes  in  and  out.  The  mar- 
gins of  the  clefts — except  the  first  or  hyomandibular  cleft — 
become  the  seat  of  a  rich  supply  of  capillary  blood-vessels, 
the  blood  of  which  obtains  oxygen  from  the  water  and  yields 
to  the  latter  its  carbon  dioxid ;  while  the  visceral  arches, 
excluding  the  first  and  second,  become  known  in  these  classes 
as  branchial  arches  from  their  producing  bony  arches  which 
support  the  branchiae  or  gills.  With  the  exceptions  noted, 
the  visceral  arches  and  clefts  with  their  capillary  plexuses 
therefore  functionate  in  these  classes  as  a  respiratory  ap- 
paratus. 

When,  in  the  course  of  evolution,  certain  of  the  verte- 
brates assume  an  aerial  existence,  in  consequence  of  which 
they  acquire  a  breathing  mechanism  adapted  to  such  a  mode 
of  life,  the  respiratory  function  of  the  clefts  or  branchiae 
ceases,  and  they  either  disappear  entirely  or  constitute  merely 
rudimentary  structures  of  the  adult.  The  so-called  clefts  in 
man  are  never  actual  openings,  the  closing  membrane  always 
being  present  (His,  Kolliker,  Piersol,  Born).  To  express  the 
morphology  of  the  visceral  clefts  briefly,  they  are  permanent 
structures  in  fishes  and  in  tailed  Amphibia ;  they  are  present 
during  the  larval  stage  of  other  Amphibia,  while  in  birds 
and  mammals  they  are  found  only  in  embryonic  life. 

The  growth  of  the  visceral  arches  and  clefts  bears  an  inti- 
mate relation  to  the  differentiation  of  the  head-  and  the  neck- 
regions  of  the  embryo.  They  first  make  their  appearance  at 
about  the  twenty-third  day  and  attain  their  greatest  develop- 
ment by  the  end  of  the  fourth  week.  Both  the  arches  and 
the  clefts  appear  earliest  and  are  best  developed  at  the  ce- 
phalic end  of  the  series,  the  fifth  arch  being  exceedingly  ill- 
defined.  During  the  fifth  week  the  obliteration  of  the  arches 
and  clefts  as  such  begins,  since  certain  of  them  become  meta- 
morphosed into  permanent  structures  while  the  remainder 
undergo  regression. 


104  TEXT-BOOK  OT  EMBRYOLOGl'. 

The  Metamorphosis  of  the  Visceral  Arches  and  Clefts. — 
The  first  visceral  arch  becomes  divided  into  an  upper  part, 
the  maxillary  arch,  and  a  lower  portion,  the  mandibular  or 
jaw-arch  (Fig.  51).  The  maxillary  arches  or  processes  of 
the  two  sides  unite,  at  their  anterior  ends,  with  the  inter- 
vening nasofrontal  process  (Fig.  56),  and  in  this  way  is  formed 
the  upper  boundary  of  the  mouth-cavity ;  the  mandibular 
processes  become  joined  with  each  other  anteriorly  and  con- 
stitute the  inferior  boundary  of  this  cavity.  The  maxillary 
processes  become  the  superior  maxillae,  Avhile  the  mandibular 
processes  become  the  lower  jaws.  The  mesodermic  core  of 
the  mass  of  tissue  constituting  the  mandibular  arch  divides 
into  three  sections,  of  which  the  two  situated  at  the  proximal 
end  of  the  arch  are  quite  small  and  give  rise  respectively  to 
the  incus  and  the  greater  part  of  the  malleus ;  the  large  distal 
segment  is  a  slender  cartilaginous  rod,  Meckel's  cartilage, 
whose  proximal  extremity  becomes  the  processus  gracilis  of 
the  malleus  (see  Chapter  XA'^IIL). 

The  second  visceral,  or  anterior  hyoid  arch  becomes  obliter- 
ated as  such,  although  a  bar  of  cartilage  which  it  contains — 
Eeichert's  cartilage — gives  rise  by  its  proximal  extremity  to 
the  stapes,^  while  the  remaining  portion  becomes  metamor- 
phosed into  the  styloid  process,  the  stylohyoid  ligament,  and 
the  lesser  cornu  of  the  hyoid  bone. 

"  The  third  or  posterior  hyoid  arch,  which  corresponds  with 
the  first  branchial  arch  of  fishes,  likewise  loses  its  identity 
as  a  surface  marking,  while  the  bar  of  cartilage  it  contains 
becomes  the  body  and  greater  cornu  of  the  hyoid  bone. 

The  fourth  and  fifth  arches  coalesce  with  the  adjacent  tis- 
sues, producing  no  special  structures. 

The  first  outer  cleft,  known  as  the  hyomandibular  cleft,  suf- 
fers obliteration  except  at  its  dorsal  extremity,  where  the 
tissues  forming  its  margins  produce  the  external  ear.  The 
remaining  three  outer  clefts  disappear  in  the  following  man- 
ner :  the  fourth  outer  cleft  becomes  covered  and  hidden  by  the 
fourth  arch,  and  the  third  and  second  clefts  are  successively 

^  Reichert,  Gegenbaur,  Hertwig ;  or  to  the  ring  of  the  stapes  according 
to  Salensky,  Gradenigo,  and  Eabl. 


THE  STAGE   OF  THE  EMBRYO. 


105 


Duried  by  the  growth  of  the  third  and  second  arches.     The 
sinking-in  of  the  lower  arches  and  clefts  (Fig.  50)  results  in 


Fig.  51. — Human  embryo  of  about  twentj'-eight  days  (His):  J- F, brain-vesicles: 
/^./■,/^./*.  cephalic,  cervical,  dorsal,  and  lumbar  flexures;  op,  eye;  ot,  otic  vesi- 
cle; ol,  olfactory  pit;  mx,  md,  maxillary  and  mandibular  processes  of  first  visceral 
arch ;  sp,  sinus  prsecervicalis  ;  W^,  h^,  heart ;  l,U,  limbs ;  als,  allantoic  stalk ;.  ch,  vil- 
lous chorion. 

the  formation  of  a  fossa  or  fissure  on  the  lateral  surface  of  the 
■neck,  the  sinus  prsecervicalis  (Fig.  51,  sp),  Avhich  subsequently 


106  TEXT-BOOK  OF  EMBRYOLOGY. 

is  made  to  disappear  by  the  coalescence  of  its  edges.  Occa- 
sionally this  sinus,  instead  of  becoming  completely  obliter- 
ated, persists,  and  the  thin  layer  of  tissue  forming  its  bottom 
ruptures — possibly  spontaneously  or  perhaps  more  probably 
as  the  result  of  exploratory  probing — constituting  the  anom- 
aly known  as  cervical  fistula.  Such  a  fistula  establishes  an 
opening  into  the  esophagus. 

The  first  inner  cleft  or  first  pharyngeal  pouch  becomes  meta- 
morphosed into  the  middle  ear  and  the  Eustachian  tube,  the 
closing  membrane,  which  separates  it  from  the  outer  cleft, 
forming  the  membrana  tympani.  The  second  pharyngeal 
pouches  produce  no  special  structures,  but  the  adjacent  tissues 
give  rise  to  the  epithelial  parts  of  the  middle  lobe  of  the  thy- 
roid body  and  to  the  posterior  third  of  the  tongue,  in  the 
manner  more  fully  indicated  on  pp.  131  and  207.  The  third 
inner  cleft  produces  the  thymus  body,  while  from  the  fourth 
results  the  lateral  lobes  of  the  thyroid  body. 

The  configuration  of  the  face,  depending  as  it  does  so  largely 
upon  the  development  of  the  boundaries  of  the  nose  and  of 
the  mouth,  is  closely  associated  with  the  growth  of  the  first 
pair  of  visceral  arches.  The  earliest  indication  of  the  mouth, 
the  oral  pit,  appears  at  about  the  twelfth  day  as  a  shallow  de- 
pression on  the  ventral  surface  of  the  embryonic  body  be- 
tween the  fore-brain  vesicle  and  the  prominence  caused  by  the 
primitive  heart  (Fig.  48,  3  to  5).  This  depression  is  deepened 
by  the  growth  of  the  tissues  surrounding  it,  as  also  by  the 
flexure  of  the  head,  which  occurs  at  the  twenty-first  day.  In 
the  third  week,  therefore,  the  oral  pit  is  a  five-sided  fossa, 
being  bounded  above  by  the  nasofrontal  process,  which  has 
grown  down  from  the  elevation  of  the  fore-brain,  laterally  by 
the  maxillary  processes,  and  below  by  the  mandibular  arches 
(Fig.  56,  A).  The  pharyngeal  membrane,  which  consists  of  op- 
posed ectoderm  and  entoderm  and  which  separates  the  primi- 
tive oral  cavity  from  the  gut-tract  (Fig.  55,  rh),  ruptures  at  the 
time  of  the  appearance  of  the  third  branchial  arch. 

By  the  end  of  the  third  week,  the  communication  betAveen 
the  yolk-sac  and  the  gut-tract  has  become  reduced  to  the 
relatively  small  vitelline  duct.     At  the  twenty-fifth  day  the 


THE  STAGE  OF  THE  FETUS.  107 

embryo  presents  a  well-developed  tail.  By  the  termination 
of  the  fourth,  week  the  yolk-sac  has  attained  its  maximum 
size,  and  the  presence  of  the  somites  is  indicated  by  trans- 
verse parallel  lines  on  the  dorsal  surface  of  the  body. 

THE  STAGE  OF  THE  FETUS. 

This  stage  comprises  the  time  between  the  beginning  of 
the  second  month  and  the  end  of  pregnancy. 

During  the  second  month  the  rate  of  growth  is  far  less 
rapid  than  in  the  preceding  stage.  The  marked  curvature 
of  the  long  axis  of  the  body  gradually  dimishes,  the  embryo 
assuming  a  more  erect  posture.  Owing  to  the  partial  disap- 
pearance of  the  cervical  flexure,  the  head  becomes  raised. 

During  the  fifth  week  the  vitelline  duct  is  seen  to  be 
long  and  slender ;  the  umbilical  cord  has  become  longer  and 
more  spiral,  and  may  contain  a  coil  of  intestine ;  the  abdo- 
men is  very  prominent,  and  in  the  neck-regi(m  is  a  charac- 
teristic dorsal  concavity.  At  this  time  also  the  nasal  pits 
become  conspicuous  as  depressions  situated  on  either  side  of 
the  nasofrontal  process  (Fig.  56).  The  nasofrontal  process 
meanwhile  undergoes  diiferentiation  into  the  globular  processes, 
which  constitute  the  inner  boundaries  of  the  nasal  pits,  and  the 
lateral  frontal  processes,  which  limit  these  depressions  exter- 


FiG.  52.— Human  embryo  of  about  six  weeks,  enlarged  three  times  (His). 

nally  and  separate  them  from  the  depressions  for  the  eyes. 
The  nasal  pits  are  still  in  communication  below  with  the 


108  TEXT-BOOK  OF  EMBRYOLOGY. 

primitive  oral  cavity.  The  lacrimal  groove  is  \yell-niarked 
at  this  stage,  and  the  external  auditory  meatus  is  indicated. 
The  mandibles  become  united  mesially  at  about  the  thirty- 
fourth  day.  The  third  and  fourth  gill-clefts  have  by  this 
time  disappeared  in  the  cervical  sinus.  The  paddle-like  limb- 
buds  have  lengthened  and  present,  at  first,  a  division  into  two 
sesfments,  of  which  the  distal  is  destined  to  become  the  iiand 
or  foot,  while  the  proximal  portion  undergoes  segmentation 
a  little  later  into  the  arm  and  forearm  or  thigh  and  leg ;  by 
the  thirty-second  day,  the  hand,  now  showing  differentiation 
into  a  thicker  proximal  and  a  thinner  terminal  part,  exhibits 
the  first  traces  of  digitation,  in  the  form  of  parallel  longi- 
tudinal markings  which  soon  become  grooves  and,  later, 
clefts.  The  development  of  the  upper  extremities  precedes 
that  of  the  lower  by  twelve  or  fourteen  days. 

During  the  sixth  week  the  head  assumes  more  nearly  its 
normal  position,  and  for  this  reason  the  apparent  length  of 
the  fetus  is  considerably  increased,  the  dorsal  concavity  in 
the  neck-region  being  ahiiost  obliterated ;  the  rudiments  of 
the  eyelids  and  of  the  concha  become  recognizable,  and  the 
various  parts  of  the  face  assume  more  definite  shape.  By 
the  fortieth  day  the  oral  cavity  has  become  separated  from 
the  nasal  pits  by  the  union  of  the  nasofrontal  process  wdth 
the  maxillary  processes,  and  the  external  boundaries  of  the 
nostrils  have  become  marked  out  by  the  meeting  of  each 
lateral  frontal  process  with  the  corresponding  maxillary 
process.  As  a  result  of  these  changes,  the  nose,  altliough 
still  very  broad,  begins  to  assume  cliaracteristic  form. 
During  this  week  also  the  fingers  are  seen  as  separate  out- 
growths, while  in  the  seventh  week  the  rudiments  of  their 
nails  become  evident. 

Toward  the  end  of  the  second  montli — about  the  fiftieth 
to  the  fifty-third  day — the  toes  are  just  beginning  to  sepa- 
rate, the  protrusion  of  the  intestine  at  the  umbilicus  is  at  its 
maximum,  tlie  palpel^ral  conjunctiva  separates  from  the  cor- 
nea, and  the  rudimentary  tail  begins  to  disappear. 

The  eighth  week  witnesses  the  total  disappearance  of 
the  free  tail,  the  formation  of  the  septum  that  divides  the 


THE  STAGE  OF  THE  FETUS.  109 

cloaca  into  the  rectum  and  the  genito-urinary  passage,  and 
the  presence  of  the  projecting  genital  tubercle  with  the  ac- 
companying genital  folds  and  genital  ridges.  The  external 
genitals  as  yet  show  no  distinction  of  sex.  From  the  end 
of  the  second  month  to  the  time  of  birth,  fetal  growth  is,  in 
great  measure,  merely  the  further  development  of  organs 
already  mapped  out ;  it  is  held  by  many  authorities,  there- 
fore, that  if  malformations  are  ever  due  to  maternal  impres- 
sions, such  impressions  could  be  operative  only  in  the  event 


/-•; 


/ 


/ 


Fig.  53. — Human  embryo  of  about  seven  weeks,  enlarged  three  times. 

of  having  been  received  prior  to  the  eighth  week  of  gesta- 
tion. 

During  the  third  month,  the  face,  although  definitely 
formed,  still  presents  thick  lips,  a  pointed  chin,  and  a  rather 
broad  and  triangular  nose.  At  this  time  the  limbs  are  well- 
formed  and  assume  a  characteristic  attitude,  and  the  fingers 
and  toes  are  provided  with  imperfect  nails.  The  external 
genitals,  which,  until  the  close  of  the  second  month,  pre- 
served the  indifferent  type,  now  begin  to  show  sexual 
distinction. 

In  the  fourth  month,  a  growth  of  fine  hair,  the  lanugo, 
appears  upan  the  scalp  and  some  other  parts  of  the  body ; 


110 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  anus  opens  ;  the  intestine  recedes  within  the  abdomen ; 
and  the  external  generative  organs  present  well-marked 
sexual  characteristics. 

The.  fifth,  month  marks  the  inauguration  of  active  fetal 
movements  and  the  appearance  of  a  more  plentiful  growth 
of  colorless  hair. 

In  the  sixth  month  the  fetal  body  becomes  coated  with 
the  vernix  caseosa,  a  modified  sebaceous  secretion  whose  func- 


FiG.  54.— Human  embryo  of  about  eight  and  a  half  weeks,  enlarged  three 

times  (His). 

tion  is  the  protection  of  the  epidermis  from  maceration  in  the 
amniotic  fluid.  The  eyebrows  and  eyelashes  also  appear 
about  this  time. 

The  seventh  month  witnesses  the  appearance  of  the  lanugo, 
or  embryonal  down,  upon  practically  the  entire  surface  of 
the  body ;  the  testes  of  the  male  fetus  are  in  the  inguinal 
canal  or  at  the  internal  abdominal  ring;  and  the  nails 
break  through  their  epidermal  covering.     Children  born  at 


THE  STAGE  OF  THE  FETUS.  Ill 

the  end  of  the  seventh  month  may  survive,  but  usually  they 
do  not. 

In  the  eighth  month  the  lanugo  begins  to  disappear. 

In  the  ninth  month  the  testicles  are  found  in  the  scro- 
tum, while,  in  the  case  of  the  female,  the  labia  majora  are  in 
contact  with  each  other.  The  contents  of  the  intestinal  canal, 
the  meconium,  consisting  of  intestinal  and  hepatic  secretions 
mingled  with  epidermal  cells  and  hairs  swallowed  by  the 
fetus,  is  now  of  a  dark  greenish  color.  The  umbilicus  is 
almost  exactly  in  the  middle  of  the  body. 

The  weight  of  the  fetus  at  full  term  is  from  3  to  3.5  kilo- 
grams (from  6  to  7  pounds),  the  average  weight  of  the  male 
child  being  about  ten  ounces  greater  than  that  of  the  female. 
While  variations  from  these  figures  are  not  uncommon,  state- 
ments of  excessive  weight  are  to  be  received  with  reservation, 
since  it  has  been  found,  upon  careful  observation  by  compe- 
tent authorities,  that  the  weight  of  a  new-born  infant  rarely 
exceeds  ten  pounds.  The  weight  of  the  child,  besides  de- 
pending upon  the  physical  condition  of  both  parents,  is  in- 
fluenced by  the  age  of  the  mother,  young  women  having  the 
smallest,  and  women  between  the  ages  of  thirtv  and  thirty- 
five  having  the  heaviest  children  ;  by  the  number  of  previous 
pregnancies,  the  weight  being  greater  with  each  succeeding 
pregnancy,  provided  the  successive  children  are  of  the  same 
sex  and  are  not  born  at  too  short  intervals ;  and  also  by  the 
weight  (Gassner)  and  height  (Frankenhaiisen)  of  the  mother, 
the  ratio  being  a  direct  one.  Minot  believes  that  these 
various  influences  operate  chiefly  by  prolonging  or  abbreviat- 
ing the  period  of  gestation,  and  that  therefore  the  variations 
in  weight  at  birth  are  referable  to  two  principal  causes — 
differences  in  the  age  at  birth,  and  variations  in  the  rate  of 
intra-uterine  growth. 

The  length  of  the  fetus  at  the  time  of  birth  is  about  50 
centimeters  (20  inches). 

The  approximate  age  of  an  embryo  or  fetus  may  be  esti- 
mated by  the  characters  peculiar  to  each  stage  as  above 
noted,  and  also  by  employing  the  rule  formulated  by  Haase. 
According  to  Haase,  up  to  the  end  of  the  fifth  month,  the 


112  TEXT-BOOK  OF  EMBRYOLOGY. 

square  of  the  age  in  months  equals  the  length  in  centimeters, 
while  after  tlie  fifth  month,  the  length  expressed  in  centi- 
meters equals  the  age  in  months  multiplied  by  five.  Thus 
a  fetus  of  four  months  would  have  a  length  of  16  centi- 
meters ;  while  one  of  six  months  would  be  30  centimeters 
long.  Hence,  the  age  in  months  is  the  square  root  of  the 
number  expressing  the  length  in  centimeters ;  or,  if  the 
length  exceeds  30  centimeters,  the  age  in  months  is  one-fifth 
of  the  length  expressed  in  centimeters. 

Reference  has  been  made  in  Chapter  I.,  page  37,  to  the 
relation  between  conception  and  menstruation,  and  to  the 
manner  of  estimating  the  age  of  the  product  of  gestation, 
based  upon  this  relation. 


CHAPTER    VIII. 

THE  DEVELOPMENT  OF  THE  CONNECTIVE  TISSUES 
OF  THE  BODY  AND  OF  THE  LYMPHATIC  SYSTEM. 

THE  CONNECTIVE  TISSUES. 

The  variously  modified  forms  of  connective  tissue  distrib- 
uted throughout  the  body,  including  such  diversified  tissues 
as  the  blood  and  the  lymph,  areolar  tissue,  fibrous  and  elastic 
tissue,  adenoid  tissue,  tendon,  cartilage,  bone,  and  dentine, 
as  well  as  the  connective-tissue  stroma  of  various  organs,  all 
result  from  alterations  affecting  the  middle  germ-layer  or 
mesoderm.  As  pointed  out  elsewhere  (Chapter  III.),  the 
inner  and  the  outer  germ-layers  are  concerned  in  producing 
the  epithelial  structures  of  the  body  (with  the  exception  of 
the  epithelium  of  the  greater  part  of  the  genital  apparatus 
and  of  the  kidney  and  ureter),  the  ectoderm  giving  rise  not 
only  to  the  epithelium  of  the  surface  of  the  body,  but  also, 
by  processes  of  infolding,  to  such  important  structures  as 
the  central  nervous  system  and  the  internal  ear,  while  the 
entoderm  differentiates  into  the  epithelial  parts  of  the  respira- 
tory and  digestive  systems  with  their  associated  glandular 
organs. 

The  proliferation  of  the  cells  of  the  mesoderm  goes  hand 
in  hand  with  the  differentiations  of  the  inner  and  outer  germ- 
layers,  so  that  even  at  an  early  stage  of  development  the 
middle  germ-layer,  besides  having  given  rise  to  the  mesothe- 
lium  of  the  body-cavity  and  to  the  primitive  segments,  con- 
stitutes a  loose  aggregation  of  cells  that  fill  the  spaces  be- 
tween the  germ-layers  and  spread  about  the  developing 
embryonic  organs.  This  primitive  relation  of  the  meso- 
dermic  tissue  foreshadows  its  future  office  as  the  supporting 
framework  not  only  of  the  body,  but  of  the  functionally 

8  113 


114  TEXT-BOOK  OF  EMBRYOLOGY. 

active  epithelial  elements  of  the  glands.  Thus,  the  indifferent 
mesodermic  tissue  that  comes  to  surround  the  notochord  and 
the  neural  canal  specializes  into  the  spinal  column  and  the 
brain-case ;  while  the  parts  of  this  tissue  into  which  protrude 
the  epithelial  evaginations  of  the  primitive  alimentary  canal 
— as,  for  example,  the  evaginations  which  are  the  beginnings 
of  its  glandular  organs,  the  liver  and  the  pancreas — become 
intimately  associated  with  these  epithelial  sacs  and  tubes  to 
constitute  the  connective-tissue  stroma  and  the  vascular  ap- 
paratus of  the  completed  glands.  All  organs  of  the  body, 
therefore,  that  have  a  connective-tissue  constituent  obtain  it 
from  the  mesoderm.  Owing  to  the  varying  degree  of  differ- 
entiation of  the  mesodermic  elements  in  different  localities 
there  are  formed  tissues  of  widely  different  character.  The 
most  important  factor  in  the  production  of  these  modifica- 
tions is  the  alteration  of  the  intet-cellular  substance,  as  to 
whether  it  remains  soft  and  homogeneous,  whether  it  ac- 
quires a  fibrillar  or  an  elastic  structure,  or  whether  it  be- 
comes dense  and  hard,  as  in  the  case  of  cartilage  and  bone. 
The  cells  undergo  comparatively  little  change,  although, 
according  to  the  kind  of  tissue  produced,  they  come  to  be 
known  respectively  as  connective-tissue  cells,  tendon-cells, 
cartilage-cells,  or  bone-cells. 

The  slightest  degree  of  specialization  results  in  the  pro- 
duction of  mucous  tissue.  In  this  case  a  reticulum  is  formed 
by  the  slender  processes  which  the  cells  acquire,  the  spaces 
of  the  meshwork  being  filled  with  the  semifluid  or  semi- 
gelatinous  intercellular  substance. 

A  further  alteration  in  the  intercellular  substance,  whereby 
it  acquires  greater  density  and  becomes  permeated  by  bun- 
dles of  fibers,  some  of  which  are  highly  elastic,  results  in  the 
formation  of  areolar  tissue.  Preponderance  of  the  non-elas- 
tic fibrous  element  produces  white  fibrous  tissue,  while  elastic 
tissue,  such  as  predominates  in  the  ligamentum  nuchse,  is 
formed  if  the  elastic  fibers  are  in  excess.  Further  increase 
in  the  density  of  the  intercellular  material,  with  its  accom- 
panying conversion  into  bundles  of  non-elastic  fibers  having  a 
characteristic  regularity  of  arrangement,  produces  the  struct- 


THE  CONNECTIVE  TISSUES.  115 

ure  of  tendon.  When  the  intercellular  substance  gives  rise 
to  a  scant  amount  of  fibrous  material  and  the  cells  become 
distended  with  oily  or  fatty  matter,  adipose  tissue  results. 

A  still  greater  degree  of  density  of  the  intercellular  sub- 
stance gives  the  matrix  of  cartilage,  the  cells  being  enclosed 
in  spaces,  the  lacunae,  as  the  cartilage-cells.  Partial  differ- 
entiation into  either  fibrous  or  elastic  bundles  confers  the 
character  of  either  fibrous  or  elastic  cartilage  upon  the 
product. 

Great  condensation  of  the  intercellular  substance  and  its 
permeation  with  salts  of  lime,  the  cells  being  fixed  in  small 
spaces,  results  in  the  production  of  osseous  tissue  (see  Chap- 
ter xvrii.). 

Blood  and  lymph  may  be  looked  upon  as  forms  of  connec- 
tive tissue  in  which  the  intercellular  substance  is  fluid,  con- 
stituting the  plasma,  the  cellular  elements  thus  remaining 
free  cells,  the  blood-  or  the  lymph-corpuscles.  The  develop- 
ment of  both  lymph  and  blood  from  the  mesodermic  elements 
serves  to  bear  out  the  comparison. 

The  endothelium  of  the  body  is  related  with  the  connective 
tissues  genetically  as  well  as  anatomically.  Reference  has 
been  made  elsewhere  to  the  changes  which  occur  in  the 
mesodermic  cells  that  bound  the  body-cavity — the  fissure 
between  the  two  layers  into  which  the  parietal  plate  of  the 
mesoderm  splits — to  constitute  the  mesothelium  of  the  body- 
cavity.  These  changes  consist  in  the  flattening  of  the  cells 
and  their  assumption  of  the  characters  of  endothelium. 
Similarly,  when  other  smaller  clefts  are  formed  in  the  meso- 
dermic tissue,  clefts  which  may  be  the  beginnings  of  small 
lymph-spaces,  or  of  blood-vessels,  or  of  bursal  or  articular 
cavities,  the  bordering  cells  of  these  cavities  also  assume  the 
endothelioid  type. 

The  mode  of  development  of  the  serous  membranes  and  of 
the  closely  allied  synovial,  bursal,  and  thecal  sacs  may  be 
inferred  from  what  has  been  said  about  the  origin  of  the 
endothelium.  The  connective- tissue  stroma  of  the  mem- 
brane, upon  which  the  endothelium  rests,  is  simply  a  con- 
densed and  differentiated  lamella  of  connective  tissue. 


116  TEXT-BOOK  OF  EMBRYOLOGY. 

THE  DEVELOPMENT  OF  THE  LYMPHATIC    SYSTEM. 

The  solid  elements  of  the  lymphatic  system — the  "  lymph- 
glands,"  the  lymph-follicles,  and  the  diffuse  adenoid  tissue — 
as  well  as  the  thymus  body  and  the  spleen,  result  from  the 
specialization  of  mesodermic  cells,  while  the  lymph-vessels 
and  the  various  lymph-spaces  of  the  economy — that  is,  the 
serous  sacs,  joint-cavities,  bursal  and  thecal  cavities,  sub- 
arachnoid and  subdural  spaces  of  the  brain  and  spinal  cord — 
are  developed  by  vacuolation  or  hollowing  out  of  the  meso- 
derm. 

Definite  knowledge  is  wanting  as  to  many  of  the  details 
of  the  genesis  of  the  lymphatic  system.  The  various  lymph- 
spaces  precede  the  vessels  and  the  adenoid  tissue  in  devel- 
opment. 

The  lymph-spaces  result  from  clefts  in  the  mesoderm,  the 
earliest  formed  and  most  conspicuous  space  of  this  sort  being 
the  body-cavity  or  coelom.  This  large  fissure  develops,  even 
before  the  differentiation  of  the  body  of  the  embryo,  by  the 
coalescence  of  numerous  small  cavities  that  appear  within 
the  middle  germ-layer.  The  body-cavity  acquires  more 
definite  boundaries  by  the  alteration  of  the  mesodermic 
cells  that  border  it  into  flattened  endothelioid  cells,  the 
mesothelium  of  the  body-cavity.  When,  in  the  progress  of 
development,  the  diaphragm  and  the  pericardium  are  formed, 
the  body-cavity  is  divided  into  the  peritoneal  cavity,  the 
pleural  sacs,  and  the  pericardium.  At  a  still  later  period, 
a  diverticulum  of  the  peritoneum  protrudes,  in  the  male  fetus, 
through  the  inguinal  canal  into  the  scrotum  to  constitute  the 
tunica  vaginalis  testis.  The  stomata  of  serous  membranes 
are  merely  so  many  apertures  of  communication  between 
the  serous  cavities,  which  are  enormous  lymph-spaces,  and 
the  lymphatic  clefts  contained  within  the  stroma  of  the 
serous  membrane,  the  clefts  themselves  being  the  begin- 
nings of  lymph-vessels. 

The  large  lymph-sacs  surrounding  the  brain  and  spinal 
cord,  the  subarachnoid  and  subdural  spaces,  as  well  as  the 
spaces  within  the  capsule  of  Tenon  and  the  sheath  of  the 


THE  DEVELOPMENT  OF  THE  LYMPHATIC  SYSTEM.     117 

optic  nerve,  and  the  perilymphatic  spaces  of  the  internal 
ear  similarly  develop  as  vacuolations  of  the  mesodermic 
tissue.  The  same  is  true  of  the  joint-cavities,  bursal  sacs, 
sheaths  of  tendons,  and  the  small  lymph-clefts  found  in  the 
areolar  tissue  and  throughout  most  organs. 

The  lymphatic  vessels  apparently  develop,  after  the  man- 
ner of  blood-vessels,  from  anastomosing  cords  of  cells  that 
are  of  direct  mesodermic  origin.  These  solid  cell-cords  are 
afterward  hollowed  out,  and  thus  become  tubes.  The  endo- 
thelial cells  lining  the  vessels  are  probably  the  metamor- 
phosed descendants  of  the  cells  of  the  original  cords.  The 
earliest  lymphatic  vessels  formed  are  the  subcutaneous  ves- 
sels, which  are  present  in  a  human  embryo  of  2-3  c.m.^  At 
a  somewhat  later  period  the  deeper  vessels  appear. 

The  lymphoid  or  adenoid  tissue  is  produced  at  a  later  date 
than  the  vessels.  Observations  upon  the  human  lymph 
nodes  seem  to  have  been  confined  to  the  inguinal  and  lum- 
bar nodes.  The  first  indication  of  an  inguinal  node  is  seen 
in  a  3  cm.  embryo,  in  the  shape  of  little  aggregations  of 
lymphoid  cells  that  have  migrated  from  the  lymphatic  cords 
or  networks  into  a  space  hollowed  out  of  the  mesoderm. 
This  nodule  of  lymphoid  cells  is  isolated  from  the  surround- 
ing mesodermic  elements  by  a  fissure  or  space  except  at  one 
point,  the  future  hilum  of  the  node,  where  strands  of  em- 
bryonal connective  tissue  connect  it  with  the  parent  meso- 
derm. The  reticulum  of  the  node  appears  later,  as  does  also 
the  capsule,  the  latter  of  which  results  from  the  condensation 
of  the  surrounding  mesoderm. 

The  development  of  the  spleen  is  considered  with  that  of 
the  alimentary  system  because  of  its  relation  to  the  evolu- 
tion of  the  peritoneum,  while  the  account  of  the  development 
of  the  thymus  will  be  found  in  the  chapter  on  the  respiratory 
system. 

^  O.  Schultze :  ' '  Grundriss  der  Entwickelungsgeschichte  des  Menschen 
und  der  Saugethiere,"   Leipzig,  1897. 


CHAPTER  IX. 

THE    DEVELOPMENT  OF  THE    FACE  AND  OF  THE 
MOUTH=CAVITY. 

The  evolution  of  the  face  depends  so  largely  upon  the 
growth  of  the  parts  concerned  especially  in  the  production 
of  the  mouth  and  nose  that  any  account  of  its  development 
must  deal  for  the  most  part  with  the  development  of  those 
structures.  In  tracing  the  earliest  stages  of  facial  growth, 
it  will  be  well  to  consider  the  face  as  a  whole  before  pro- 
ceeding to  a  detailed  description  of  its  several  parts.  If  we 
seek  the  principles  underlying  the  conformation  of  the  face, 
we  shall  find  that  its  apertures  and  chief  cavities  are  merely 
so  many  provisions  for  bringing  the  central  nervous  system 
and  the  alimentary  tract  into  relation  with  the  outside  world. 
It  will  be  seen,  for  example,  that  certain  small  depressions 
appear  upon  the  surface  ;  that  one  of  these,  which  is  destined 
to  become  the  mouth  and  the  respiratory  part  of  the  nasal 
cavities,  assumes  relationship  with  the  alimentary  tract  and 
with  its  offshoot,  the  respiratory  system ;  that  other  depres- 
sions, which  subsequently  develop  into  the  olfactory  parts 
of  the  nasal  chambers,  come  into  relation  with  outgrowths 
from  the  brain,  the  olfactory  bulbs ;  and  that  still  another 
surface-invagination  becomes  the  lens-vesicle,  which  likewise 
meets  with  an  outgrowth  from  the  brain  to  become  a  part  of 
a  peripheral  sense-organ,  the  eye. 

The  first  step  in  the  differentiation  of  the  face  is  the  for- 
mation of  the  oral  plate,  the  earliest  indication  of  the  future 
mouth.  The  oral  plate  appears  on  the  twelfth  day,  and  con- 
sists of  a  small  area  of  ectoderm  and  entoderm,  the  meso- 
derm being  absent.     It  is  situated  on  the  ventral  surface  of 

118 


DEVELOPMENT  OF  FACE  AND   OF  MOUTH-CAVITY.    119 

the  head-end  of  the  embryo,  which  already  presents  the 
enlargement  of  the  cerebral  vesicles.  The  oral  plate  becomes 
relatively  depressed  by  the  upgrowth  of  the  surrounding 
tissues,  the  fossa  thus  produced  constituting  the  oral  pit  or 
stomodseum  (Fig.  46).  The  oral  plate  is  now  the  pharyngeal 
membrane  (Fig.  55).     Reference  to  the  sagittal  section  will 


Fig.  55.— Median  section  through  the  head  of  an  embryo  rabbit  6  mm.  long 
(after  Mihalkovics) :  rh,  membrane  between  stomodaeum  and  fore-gut,  pharyngeal 
membrane  (Rachenhautj;  hp,  place  from  which  the  hypophysis  is  developed;  h, 
heart;  M,  lumen  of  fore-gut;  ch,  chorda;  v,  ventricle  of  the  cerebrum;  -y',  third 
ventricle,  that  of  the  between-brain  (thalamencephalon) ;  «*,  fourth  ventricle,  that 
of  the  hind-brain  and  after-brain  (epencephalon  and  metencephalon,  or  medulla 
oblongata) ;  cfc,  central  canal  of  the  spinal  cord. 


show  that  the  oral  pit  corresponds  in  position  to  the  head- 
end of  the  gut-tract.  The  formation  of  the  pit  is,  in  effect, 
a  pushing-in  of  the  surface  ectoderm  to  meet  the  alimentary 
entoderm. 

A  second  important  factor  in  the  development  of  the  face 
is  the  appearance  of  the  first  and  second  visceral  arches, 
which  occurs  in  the  third  week.  As  pointed  out  in  a  pre- 
ceding section,  the  first  visceral  arch  divides  into  the  mandi- 
bular arch  and  the  maxillary  process  (Fig.  51),  the  latter 
being  the  smaller  and  appearing  to  spring  from  the  mandi- 
bular arch.  Both  the  maxillary  processes  and  the  mandi- 
bular arches  grow  toward  the  median  line  of  the  ventral 
surface  of  the  body.     Owing  to  the  growth  of  these  struct- 


120  TEXT-BOOK  OF  EMBRYOLOGY. 

ures  and  to  the  sharp  flexion  of  the  head  and  neck  that 
occurs  between  the  twenty-first  and  the  twenty-third  day, 
the  oral  pit  becomes  very  much  deeper  and  acquires  more 
definite  boundaries.  During  the  third  week  it  is  a  fiDSsa  of 
pentagonal  outline.  Its  upper  boundary  is  formed  by  the 
unpaired  nasofrontal  or  nasal  process  (Fig.  56,  A),  which  is 
essentially  a  thickening  on  the  ventral  wall  of  the  fore- 
brain  vesicle,  brought  into  close  relation  with  the  fossa  by 
the  flexion  above  referred  to.  The  lower  boundary  is  formed 
by  the  mandibular  arches,  while  the  lateral  extent  of  the  fossa 
is  limited  by  the  maxillary  process  of  each  side. 

Soon  after  the  appearance  of  the  oral  pit,  the  future  nares  are 
foreshadowed  by  the  development  of  the  two  olfactory  plates, 
situated  one  on  each  side  of  the  nasofrontal  process,  widely 
separated  from  each  other.  These  epithelial  areas,  which 
soon  become  depressions,  the  nasal  pits,  are  closely  united 
with  the  wall  of  the  fore-brain  vesicle  from  the  first ;  they 
develop  subsequently  into  that  part  of  the  nasal  mucous 
membrane  which  is  concerned  especially  with  the  sense  of 
smell.  This  fact  becomes  very  significant  when  it  is  remem- 
bered that  the  olfactory  bulbs,  with  which  the  olfactory  epi- 
thelium assumes  intimate  relationship,  are  outgrowths  from 
the  brain. 

The  nasofrontal  process,  during  the  fifth  week,  becomes 
much  thickened  along  its  lateral  margins,  forming  thus  the 
globular  processes  (Fig.  56,  A),  which  constitute  the  inner 
boundaries  of  the  nasal  pits.  At  the  same  time,  there  grow 
downward  and  forward  from  the  nasofrontal  process  two 
ridges,  one  on  each  side,  the  lateral  frontal  processes,  which 
form  the  outer  boundaries  of  the  nasal  pits  (Fig.  56,  A). 
In  this  manner  the  pits  become  much  increased  in  depth. 
The  lateral  frontal  process  projects  between  the  nasal  pit 
and  the  maxillary  process,  its  line  of  contact  with  the  latter 
structure  being  marked  by  a  groove,  the  naso-optic  furrow  or 
lacrimal  groove.  This  groove  later  completely  disappears ; 
it  is  of  importance,  however,  as  indicating  the  position  of 
the  now  developing  nasal  duct,  which  Avill  be  referred  to 
hereafter.     The  nasal  pits  are  widely  in  communication  with 


DEVELOPMENT  OF  FACE  AND   OF  MOUTH-CAVITY.   121 

the  cavity  of  the  primitive  mouth.  About  the  fortieth  day, 
however,  the  extremities  of  the  maxillary  processes  have 
grown  so  far  toward  the  median  line  that  they  have  met 


f  A 


/%    ^^ 


Fig.  56.— Development  of  the  face  of  the  human  embryo  (His) :  A,  embryo  of 
about  twenty-nine  days.  The  nasofrontal  plate  differentiating  into  processus 
globulares,  toward  which  the  maxillary  processes  of  first  visceral  arch  are  extend- 
ing. B,  embryo  of  about  thirty-four  days :  the  globular,  lateral  frontal,  and  max- 
illary processes  are  in  apposition ;  the  primitive  opening  is  now  better  defined.  C, 
embryo  of  about  the  eighth  week:  immediate  boundaries  of  mouth  are  more  defi- 
nite and  the  nasal  orifices  are  partly  formed,  external  ear  appearing.  D,  embryo 
at  end  of  second  month. 

and  united  with  the  lateral  frontal  processes  and  with  the 
nasofrontal  process  (Fig.  56,  B  and  C).  In  this  manner 
the  nasal  pits  become  separated  from  the  oral  fossa,  each  of 
these  openings  acquiring  more   definite   boundaries.     It  is 


122  TEXT-BOOK  OF  EMBRYOLOGY. 

apparent  from  this  description  that  the  upper  boundary  of 
the  primitive  oral  cavity  is  not  identical  with  that  of  the 
adult  mouth.  The  nasofrontal  process  is  the  forerunner  of 
the  intermaxillary  portion  of  the  upper  jaw,  including  the 
corresponding  part  of  the  upper  lip  and  of  the  nasal  sep- 
tum and  bridge  of  the  nose.^  The  lateral  frontal  process 
becomes  the  wing  of  the  nose.  By  the  completion  of  the 
changes  here  noted  the  face  acquires  more  distinctive  form. 
It  will  be  seen  that  the  upper  jaw  proper  results  from  the 
metamorphosis  of  the  maxillary  processes.  The  manner  in 
which  its  sinus,  the  antrum  of  Highmore,  is  added,  as  well 
as  the  ossification  of  the  jaw,  will  be  considered  hereafter. 

The  development  of  the  eye  will  be  described  in  con- 
nection with  that  of  the  sense-organs.  In  so  far  as  the  eyes 
have  relation  to  the  external  form  of  the  face,  it  will  be  suf- 
ficient to  say  that  the  surface  ectoderm  is  invaginated  in  the 
fourth  week  to  form  the  lens-vesicle,  this  sac,  which  gives 
rise  to  the  crystalline  lens,  being  covered  by  two  little  folds 
of  ectoderm,  the  primitive  eyelids;  that  the  organ  is  situated 
on  the  side  of  the  head,  in  marked  contrast  to  its  position  in 
the  mature  state ;  and  that  the  naso-optic  furrow,  previously 
referred  to,  passes  from  the  inner  angle  of  the  eye  toward 
the  wing  of  the  nose.  The  development  of  the  face  having 
been  pointed  out  in  a  general  way,  the  individual  parts  may 
be  considered  separately. 

THE  MOUTH. 

To  review  briefly,  for  the  sake  of  convenience  and  clear- 
ness, the  earlier  history  of  the  development  of  the  mouth, 
we  find  the  first  step  to  be  the  appearance,  at  the  twelfth 
day,  of  the  oral  plate.  By  the  enlargement  of  the  anterior 
end  of  the  neural  tube  to  form  the  cerebral  vesicles,  and  by 

'  Hare-lip  is  the  deformity  resulting  from  failure  of  union  between  the 
nasofrontal  and  the  maxillary  processes.  Since  the  nasofrontal  process  is 
an  unpaired  structure,  in  which  develop  the  intermaxillary  bones,  and 
which  unites  on  either  side  with  the  corresponding  maxillary  process^ — the 
latter  being  the  forerunner  of  the  upper  maxilla  proper — we  have  an 
explanation  of  the  lateral  position  of  hare-lip.  This  defect  may  be,  of 
course,  either  unilateral  or  bilateral. 


THE  MOUTH.  123 

the  development  of  the  visceral  arches,  this  area  becomes  a 
depression,  the  oral  pit.  The  pit  is  at  first  bounded  caudad 
by  the  cardiac  prominence  and  cephalad  by  the  fore-brain 
vesicle  (Fig.  46).  In  the  third  week  the  oral  pit  becomes  a 
five-sided  fossa,  owing  to  the  growth  of  several  new  struct- 
ures. These  are  the  unpaired  nasofrontal  process,  which 
bounds  the  fossa  above,  the  mandibular  arches,  which  bound 
it  below,  and  the  maxillary  processes,  which  form  the  lateral 
boundaries  (Fig.  56).  The  mandibular  arches  do  not  actually 
unite  with  each  other  until  the  thirty-fifth  day.  The  angle 
between  the  maxillary  process  and  the  mandibular  arch 
corresponds  to  the  angle  of  the  future  mouth.  In  the  sixth 
week — about  the  fortieth  day — the  oral  fossa  acquires  a  new 
upper  boundary,  which  separates  it  from  the  nasal  pits,  by 
the  growth  of  the  maxillary  and  lateral  nasal  processes 
toward  the  median  line  to  unite  with  the  nasofrontal 
process. 

The  primitive  oral  cavity,  as  before  mentioned,  is  at  first 
separated  from  the  gut-tract  by  the  pharyngeal  membrane 
(Fig.  55).  This  structure  ruptures  at  some  time  during  the 
fourth  week,  thus  bringing  the  mouth  into  communication 
with  the  upper  end  of  the  gut-tract.  The  exact  location  of 
the  pharyngeal  membrane  with  reference  to  the  adult  pharynx 
is  somewhat  difficult  to  define ;  it  is  certain,  however,  that 
the  primitive  mouth  includes  more  than  the  limits  of  the 
adult  oral  cavity,  comprising,  in  addition  to  the  latter,  the 
anterior  part  of  the  adult  pharynx.  Reference  to  a  sagittal 
section,  as  in  Fig.  55,  shows  the  relation  of  the  oropharyngeal 
cavity  to  the  brain-case ;  in  the  tissue  separating  the  two  the 
floor  of  the  cranium  is  subsequently  formed.  A  little  evagi- 
nation  from  a  point  (hp,  Fig.  55)  in  the  back  part  of  the 
primitive  oral  cavity  becomes  the  anterior  portion  of  the 
pituitary  body  or  hypophysis,  the  posterior  lobe  of  which 
develops  as  an  evagination  from  the  floor  of  the  primary 
fore-brain  vesicle.  With  the  development  of  the  floor  of 
the  cranium,  the  hypophysis  becomes  entirely  isolated  from 
the  oral  cavity.  A  little  pouch  or  recess  usually  demonstrable 
in  the  median  line  of  the  roof  of  the  pharynx  of  the  child, 


124  TEXT-BOOK  OF  EMBRYOLOGY. 

though  not  always  present  in  the  adult,  is  the  persistent 
pharyngeal  end  of  the  diverticulum  that  forms  the  hypo- 
physis ;  it  is  known  as  the  pharyngeal  bursa  or  Rathke's 
pocket. 

Very  soon  after  the  formation  of  the  upper  jaw  in  the 
manner  above  described,  the  oral  surface  of  the  jaw  jjresents 
two  parallel  ridges.  Of  these,  the  outer,  which  is  the  larger, 
develops  into  the  upper  lip,  while  the  inner  smaller  ridge  be- 
comes the  gum.  The  lip  and  gum  of  the  lower  jaw  are  pro- 
duced similarly,  at  the  same  time  or  a  little  later.  So  far, 
the  only  demarcation  between  the  mouth  and  the  nasal 
cavity  is  furnished  by  the  tissue  representing  the  united 
nasofrontal,  lateral  nasal,  and  maxillary  processes,  the  nares 
opening  widely  into  the  cavity  of  the  mouth  posterior  to  this 
partition. 

The  formation  of  the  palate,  however,  eifects  a  separation 
between  the  two  that  gives  to  each  space  its  permanent  limita- 
tions. On  the  inner  or  oral  surface  of  the  upper  jaw  two 
shelf-like  projections  appear,  one  on  each  side,  wdiich  are 
the  rudiments  of  the  future  palate.  These  gradually  grow 
toward  each  other,  the  tongue,  which  has  meanwhile  been 
developing,  projecting  upward  between  them.  In  the  eighth 
week,  union  of  these  two  lateral  halves  of  the  palate  begins 
at  their  anterior  extremities.  By  the  ninth  week  union  has 
taken  place  as  far  back  as  the  extent  of  the  future  hard 
palate,  and,  by  the  eleventh  week,  the  constituent  halves  of 
the  soft  palate  have  united  also.  As  these  two  halves  ap- 
proach each  other  the  tongue  recedes  from  betw^een  them, 
owing  to  the  growth  of  the  lower  jaw,  so  that,  when  union 
occurs,  that  organ  occupies  its  normal  position  under  the 
palate.  Osseous  formation  witliin  the  soft  tissue  first  formed 
produces  the  palate  processes  of  the  superior  maxillte  and  of 
the  palate  bones,  which  processes  collectively  constitute  the 
hard  palate  of  the  adult.  The  intermaxillary  bones  are 
formed  within  the  primitive  partition  between  the  mouth  and 
the  nares.  The  completion  of  the  palate  definitely  marks 
off  the  nasal  chambers  from  the  mouth,  thus  dividing  the 
early  oral  cavity  into  a  lower  space,  the  true  mouth,  and  an 


THE  MOUTH.  125 

upper  region,  which  is  essentially  a  part  of  the  respiratory 
system. 

The  uvula  appears  during  the  latter  half  of  the  third 
month  as  a  small  protuberance  on  the  posterior  edge  of  the 
soft  palate.^ 

The  Teeth. — The  teeth,  morphologically  considered,  are 
calcified  papillae  of  the  skin,  capped  by  a  layer  of  peculiarly 
modified  and  calcified  cells  of  the  epidermis.  Although  in 
man  and  the  higher  mammals  the  teeth  are  found  only  upon 
the  gums,  in  certain  lower  types  they  have  a  much  wider 
distribution,  occurring  upon  the  roof  and  floor  of  the  mouth 
and  in  the  pharynx,  and  also,  in  selachians,  upon  the  general 
skin-surface,  in  which  latter  case  they  are  so  modified  as  to 
constitute  scales. 

The  dentine  and  cementum  of  the  tooth,  as  well  as  its  pulp, 
are  derived  from  the  mesoderm ;  the  enamel  is  a  direct  de- 
rivative of  the  overlying  ectodermic  epithelium.  Mammals 
are  said  to  be  diphyodont,  since  they  develop  two  sets  of 
teeth ;  while  such  groups  as  sharks,  which  continue  to  pro- 
duce and  lose  new  teeth  throughout  life,  are  denominated 
polyphyodont. 

The  development  of  the  teeth  is  inaugurated  in  the  sixth 
week  of  embryonic  life  by  the  multiplication  of  the  epidermal 
cells  covering  the  surface  of  the  gums  to  form  a  linear  ridge. 
The  growth  of  the  ridge  is  away  from  the  surface,  so  that 
the  new  structure  projects  into  the  underlying  mesoderm. 
This  horseshoe-shaped  ridge,  which  corresponds  in  direction 
and  extent  to  the  line  of  the  gum,  subdivides  into  two 
parallel  ridges,  of  which  the  outer  marks  the  position  of  the 
future  groove  between  the  gum  and  the  lip  ;  the  inner  is  the 
dental  shelf  or  dental  ridge,  which  must  be  regarded  as  the 
earliest  indication  of  the  future  teeth.  The  dental  shelf 
extends  into  the  underlying  mesodermic  tissue,  not  directly 

^  Deficiency  of  union  of  the  halves  of  the  palate,  resulting  in  a  median 
fissure,  constitutes  the  deformity,  cleft  -palate.  This  deficiency  may  be 
limited  to  the  hard  or  to  the  soft  palate,  or  it  may  afiect  both,  or  it  may 
be  seen  in  the  uvula,  either  alone — deft  or  bifid  uvula — or  in  conjunction 
with  cleft  palate. 


126 


TEXT-BOOK  OF  EMBRYOLOGY. 


downward  but  in  an  oblique  direction  toward  the  inner  or 
lingual  surface  of  the  gum.  While  the  dental  shelf  is  grow- 
ing, its  line  of  connection  with  the  surface  ectoderm  is 
marked  by  the  superficial  dental  groove,  which  at  one  time 
was  looked  upon  as  being  the  first  evidence  of  tooth-forma- 
tion. 

Upon  the  side  of  the  dental  shelf  opposite  the  free  or 
oral  surface,  individual  protuberances  develop,  corresponding 
in  number  to  that  of  the  teeth  of  the  temporary  set — ten  for 
each  jaw.  Each  little  projection  consists  of  a  mass  of  ecto- 
dermic  cells,  which  soon  becomes  expanded  at  its  deep  ex- 
tremity, becoming  thus  club-shaped  and  later  flask-shaped, 
and  which  is  called  the  enamel- sac  or  primitive  enamel-germ, 
since  the  enamel  of  the  tooth  is  developed  from  it  (Fig.  57). 


Fig.  57.— Three  successive  stages  in  the  development  of  a  tooth-germ  of  a  pig 
embryo  (after  Frey  and  Thiersch) :  a,  b,  c,  layers  of  thickened  oral  epithelium, 
Showing  dental  groove  on  surface  in  3;  e,  enamel  organ;  /,  dental  papilla;  g,  h, 
internal  and  external  layers  of  the  follicle  wall;  i,  blood-vessel;  k,  maxilla;  d, 
epithelial  ingrowth,  the  end  of  which  expands  into  the  enamel-sac. 


Meanwhile  the  continuity  of  the  original  dental  shelf  is 
broken  by  the  disappearance  of  the  cells  in  the  intervals 
between  the  individual  enamel-germs,  each  germ  becoming 
thereby  isolated  from  its  neighbors.     The  neck  of  the  flask- 


THE  MOUTH.  127 

shaped  enamel-germ  becomes  reduced  to  a  slender  strand  of 
cells  and  finally  disappears,  so  that  there  is  no  longer  any 
connection  between  the  enamel-sac  and  the  ectodermic  cells 
of  the  free  surface  of  the  gum.  While  the  enamel-sacs  for 
the  temporary  teeth  are  growing  in  this  manner,  the  corre- 
sponding structures  for  the  teeth  of  the  permanent  dentition 
bud  from  the  inner  side  of  the  dental  shelf — that  is,  the  side 
looking  toward  the  tongue — except  those  for  the  three  per- 
manent molars,  which  grow  backward  toward  the  articulation 
of  the  jaw  from  the  position  of  the  second  temporary  molar. 

As  the  enamel-germs  grow  downward  into  the  mesodermic 
tissue,  the  latter  sends  up  a  number  of  conical  projections, 
the  dental  papillae,  one  for  each  enamel-organ.  This  dental 
papilla,  of  mesodermic  origin,  is  the  parent  of  the  dentine 
and  of  the  pulp  of  the  tooth.  When  the  dental  papilla  and 
the  enamel-sac  meet,  the  sac  becomes  invaginated,  its  under 
surface  assuming  a  concave  form.  The  enamel-sac  at  this 
stage  therefore  is  a  double-walled  cup  which  caps  the  dental 
papilla.  It  is  at  about  this  time  that  the  connection  of  the 
enamel-organ  with  the  surface  ectoderm  is  lost. 

The  further  evolution  of  the  enamel-organ  consists  essen- 
tially in  the  arrangement  of  its  constituent  cells  into  three 
layers  and  the  formation,  by  the  deepest  of  these  three  layers, 
of  the  special  elements  of  the  fully-developed  enamel — the 
enamel-prisms.  The  most  superficial  stratum  of  the  enamel- 
organ  is  composed  of  low  columnar  or  polyhedral  cells ;  the 
deepest  layer,  that  nearest  the  papilla,  the  so-called  mem- 
brana  adamantina,  consists  of  beautifully  regular  columnar 
cells,  the  amelolilasts  or  adamantoblasts  ;  between  the  two  is 
a  group  of  less  characteristic  epithelial  elements.  The  cells 
of  the  deep  layer,  the  enamel-cells,  are  alone  concerned  in 
the  production  of  the  enamel.  The  enamel-organ  for  a  time 
covers  the  entire  dental  papilla.  During  the  course  of  de- 
velopment, however,  the  growth  of  that  part  of  it  covering 
the  future  root  of  the  tooth  aborts,  leaving  the  crown  alone 
covered  with  the  enamel. 

The  first  step  in  the  formation  of  the  enamel-prisms  by 
the  enamel-cells  is  that  the  protoplasm  of  the  deep  extremity 


128 


TEXT-BOOK  OF  EMBRYOLOGY. 


■^fiiilMi^ii^ 


of  each  cell  becomes  homogeneous,  and  a  tuft  develops  on 
the  end  of  the  cell,  projecting  toward  the  papilla.     By  the 

calcification  of  this  tuft  the  for- 
mation of  an  enamel-prism  is 
begun  (Fig.  58).  The  process 
of  calcification  continues  to  ad- 
vance from  the  deep  or  papillary 
aspect  of  the  enamel-organ  toward 
the  surface.  From  this  it  comes 
about  that  the  newest  enamel  is 
next  to  the  enamel-cells,  or,  in 
other  words,  nearest  the  surface, 
and  also  that  the  enamel-prisms 
are  arranged  in  a  direction  gen- 
erally vertical  to  the  free  surface 
of  the  tooth.  The  formation  of  the 
enamel  of  the  milk-teeth  begins  in 
the  latter  part  of  the  fourth  month. 
The  middle  layer  of  the  enamel- 
organ  becomes  greatly  altered  in 
constitution,  owing  to  the  accu- 
mulation of  fluid  and  to  the  re- 
duction of  its  cells  to  the  form 
of  thin  plates,  the  appearance 
being  rather  that  of  connective 
tissue  than  of  an  epithelial  structure.  The  superficial  layer 
of  cells  undergoes  atrophy,  their  exact  fate  not  being  known. 
The  atrophic  remnant  of  the  enamel-organ  is  found  upon 
the  free  surface  of  the  tooth  for  a  variable  time  after  its 
eruption,  constituting  the  membrane  of  Nasmyth. 

The  dental  papilla  has  been  referred  to  as  the  structure 
that  gives  rise  to  the  dentine.  It  originates  from  active 
multiplication  of  the  mesodermic  cells.  The  number  of 
papillae  corresponds  to  the  number  of  enamel-organs.  As 
the  papilla  grows  toward  the  enamel-organ  it  early  acquires 
vascularity.  The  shape  of  the  papilla,  whether  that  of  an 
incisor,  of  a  canine,  or  of  a  molar  tooth,  is  determined  by 
the  shape  which  the  enamel-organ  assumes.     The  connective- 


FiG.  58.— Semi-diagrammatic  fig- 
ure showing  the  several  parts  of 
a  calcifying  enamel-organ  tTour- 
neux) :  1,  central  cells  of  enamel- 
organ  ;  2  and  3,  cells  of  inner  layer 
of  enamel-organ,  3  being  the  en- 
amel cells ;  4,  zone  of  young  en- 
amel ;  5,  enamel  prisms ;  6,  young 
dentine  traversed  by  the  dentinal 
fibers;  7,  odontoblasts;  8,  central 
tissue  of  dental  papilla. 


THE  MOUTH.  129 

tissue  cells  upon  the  surface  of  the  papilla  assume  distinctive 
character,  becoming  large  and  branched,  and  constitute  the 
so-called  odontoblasts  (Fig.  58).  They  are  virtually  modi- 
fied osteoblasts.  Forming  a  continuous  layer,  they  have 
been  styled  the  membrana  eboris.  Between  this  layer  of 
odontoblasts  and  the  enamel-organ  a  layer  of  intercellular 
substance  appears,  the  membrana  praeformativa.  The  odon- 
toblasts now  send  out  processes  toward  the  enamel-organ, 
which  are  known  as  the  dental  processes.  Calcification 
begins  upon  the  surface  of  the  papilla  and  progresses  toward 
its  center,  but  is  not  complete.  Small  uncalcified  areas,  cor- 
responding to  the  globular  spaces  of  the  completed  tooth, 
remain  next  the  enamel.  The  dental  processes  likewise  fail 
to  become  calcified,  and  these  are  the  adult  dentinal  fibers 
occupying  the  dentinal  tubules  of  the  finished  dentine.  The 
odontoblasts  continue  the  formation  of  dentine  until  the  den- 
tal papilla  is  entirely  surrounded  by  it.  What  remains  of 
the  papilla,  upon  the  completion  of  the  tooth,  constitutes  the 
pulp,  a  highly  vascular  connective-tissue  substance  support- 
ing upon  its  surface  the  odontoblasts.  The  deposition  of 
dentine  begins  in  the  latter  part  of  the  fourth  month. 

During  the  metamorphosis  of  the  dental  papilla  the  meso- 
dermic  tissue  immediately  surrounding  it  undergoes  slight 
condensation  to  form  the  follicle  of  the  developing  tooth. 
As  the  enamel-organ  recedes  from  the  surface,  the  follicle 
increases  in  extent  to  such  a  degree  as  to  envelop  the  entire 
rudimentary  tooth.  Only  that  part  of  the  follicle  which 
covers  the  future  root  of  the  tooth  is  of  subsequent  import- 
ance, however ;  undergoing  partial  transformation  into  true 
bony  tissue,  it  gives  rise  to  the  cementum  or  crusta  petrosa,, 
while  the  unossified  external  fibrous  layer  constitutes  the 
lining  periosteum  of  the  alveolus  (Fig.  57). 

The  development  of  the  permanent  teeth  is  precisely  analo- 
gous to  that  of  the  milk-teeth.  The  enamel-germs  for  the 
permanent  teeth,  with  the  exception  of  the  molars,  bud  from 
the  lingual  side  of  the  dental  shelf  in  the  seventeenth  iveek 
(Fig.  59),  the  germ  for  the  first  permanent  molar  appearing 
about  a  week  earlier  at  the  posterior  extremity  of  the  dental 


130 


TEXT-BOOK  OF  EMBRYOLOGY. 


shelf  after  the  manner  of  a  milk-tooth.  The  germ  for  the 
second  molar  buds  from  the  neck  of  the  first  molar  in 
the  third  month  after  birth,  while  that  of  the  third  molar, 
the  wisdom  tooth,  springs  from  the  neck  of  the  second  about 
the  third  year.     At  birth,  therefore,  the  gums  contain  the 


Fig.  59.— Cross-section  of  the  lower  jaw  of  a  cat  embryo,  showing  the  enamel- 
germs  of  a  milk-tooth  and  of  a  permanent  tooth  (from  Bonnet,  after  Kolliker) : 
e,  thickened  oral  epithelium ;  so,  enamel-organ  of  permanent  tooth,  which  has 
grown  out  at  ss  from  the  neck  (s)  of  the  enamel-sac  of  a  milk-tooth;  mi,  lower  jaw; 
m,  Meckel's  cartilage. 


two  sets  of  teeth  except  the  second  and  third  permanent 
molars. 

The  eruption  of  the  temporary  teeth  begins  usually  at  about 
five  and  a  half  months  after  birth  with  the  appearance  of  the 
central  incisors,  and  is  complete  at  from  eighteen  to  thirty- 
six  months,  when  the  second  molars  are  cut.  The  first  teeth 
of  the  permanent  dentition  are  the  first  molars,  which  are 
erupted  at  about  the  sixth  year.  The  accompanying  table 
shows  the  time  and  the  order  of  eruption  of  the  teeth  : 


THE  MOUTH.  131 

Tempokary  Dentition. 

Central  incisors 5^  to    7  months. 

Lateral  incisors 7    to  10  months. 

First  molars 12    to  14  months. 

Canines 14    to  20  mouths. 

Second  molars 18    to  36  months. 

Permanent  Dentition. 

First  molars 6th  year. 

Central  incisors 7th  year. 

Lateral  incisors 8th  year. 

First  premolars 9th  year. 

Second  premolars 10th  year. 

Canines 11th  to  12th  year. 

Second  molars 12th  to  13th  year. 

Third  molars  (wisdom  teeth) 17th  to  21st  year. 

The  Salivary  Glands. — The  salivary  glands,  which  in 
mammals  consist  of  three  pairs,  the  parotid,  the  submaxillary, 
and  the  sublingual,  develop  as  outgrowths  of  epithelium 
from  the  lining  mucous  membrane  of  the  mouth.  The 
epithelial  elements  of  the  glands  are  therefore  of  ectodermic 
origin.  The  growth  of  the  submaxillary  gland  begins  in  the 
sixth  week,  that  of  the  parotid  in  the  eighth  week.  Each 
epithelial  outgrowth  is  at  first  a  solid  cylinder,  which  under- 
goes repeated  branching  and  acquires  a  connective-tissue 
framework  and  capsule  from  the  surrounding  mesoderm.  It 
is  not  until  the  middle  of  the  fifth  month  that  the  lumen  of 
the  gland  appears.  This  is  brought  about  by  the  moving 
apart  of  the  epithelial  cells  composing  the  cylinders  and 
their  branches.  The  main  duct  of  the  gland  first  becomes 
hollow,  then  its  branches,  and  finally  the  lumina  of  the 
alveoli  make  their  appearance.  The  respective  sites  from 
which  the  several  glands  grow  correspond  in  a  general  way 
to  the  positions  at  which  the  ducts  of  the  adult  glands  open 
into  the  mouth-cavity. 

The  Tongue. — Although  the  tongue  originates  from 
tissues  belonging  really  to  the  walls  of  the  pharynx,  its  de- 
velopment may  be  conveniently  considered  in  connection 
with  that  of  the  mouth  because  of  its  relations  in  the  mature 
organism.     This  organ,  composed  chiefly  of  muscular  sub- 


132 


TEXT-BOOK  OF  EMBRYOLOGY. 


stance,  is  formed  from  three  originally  separate  parts,  an 
anterior  unpaired  fundament,  and  two  posterior  bilaterally 
symmetrical  segments.  The  line  of  union  of  these  three 
parts  is  indicated  approximately  in  the  adult  organ  by  the 
V-shaped  row  of  circumvallate  papillae  on  the  dorsum  of  the 
tongue.  The  anterior  part  of  the  tongue  develops  from  a  small 
unpaired  tubercle,  the  tuberculum  impar,  which  grows  from 
the  median  line  of  the  floor  or  anterior  wall  of  the  pharynx 
between  the  first,  or  mandibular,  and  the  second,  or  hyoid, 
arch  (Fig.  60,  6).    The  posterior  segment  of  the  tongue  results 


Fig.  60.— Coronal  sections  of  two  human  embryos,  showing  ventral  wall  of 
pharyngeal  end  of  gut-tract  from  behind  (from  Tourneux,  after  His).  A,  from 
embryo  of  3.2  mm.;  B,  of  4.25  mm.  (about  25  to  30  days).  I,  II,  III,  IV,  outer  vis- 
ceral furrows  ;  V,  sinus  prsecervicalis,  comprising  third  and  fourth  outer  furrows ; 
1,  S,  3,  If,  visceral  arches,  each  with  its  visceral-arch  vessel ;  6,  tuberculum  impar ; 
7,  orifice  of  larynx  ;  8,  pulmonary  evagination. 

from  the  growing  together  of  two  lateral  halves,  which  develop 
from  the  anterolateral  walls  of  the  pharynx  at  the  position 
of  the  second  and  third  visceral  arches,  but  not  from  the 
arches  themselves.  These  ridges  are  sometimes  described  as 
the  fused  anterior  (ventral)  extremities  of  the  arches  just 
mentioned.  The  unpaired  tubercle  increases  in  size  to  such 
an  extent  as  to  constitute  the  major  part  of  the  organ.  In  the 
median  line  of  the  anterior  wall  of  the  pharynx,  immediately 
behind  the  tuberculum  impar,  the  epithelial  lining  of  this 
€avity  pouches  forward  and  downward  to  develop  later  into 
the  middle  lobe  of  the  thyroid  body.  As  the  ridges  which 
are  to  form  the  posterior  part  of  the  tongue  lie  laterally  and 


THE  DEVELOPMENT  OF  THE  NOSE.  133 

posteriorly  to  this  median  evagination,  they  completely  en- 
close it  in  the  process  of  fusing  with  each  other  and  with  the 
anterior  tubercle.  In  this  manner  a  canal  or  duct  is  formed 
leading  from  the  surface  of  the  tongue  at  the  angle  of  junc- 
tion of  its  three  segments  down  to  the  middle  lobe  of  the 
thyroid  body,  the  latter  meanwhile  having  descended  from 
its  original  position.  This  canal  is  the  thyroglossal  duct  or 
canal  of  His.  During  the  further  progress  of  development, 
the  canal  suffers  obliteration,  its  only  vestige  being  the  orifice, 
which  is  known  as  the  foramen  caecum  of  adult  anatomy. 

The  papillse  of  the  tongue  are  found  exclusively  on  the 
part  derived  from  the  tuberculum  impar ;  the  line  of  union 
between  the  anterior  and  posterior  parts  lies  therefore  behind 
the  row  of  circumvallate  papillse.  The  papillse  begin  to 
make  their  appearance  as  early  as  the  beginning  of  the  third 
month. 

Prior  to  the  union  of  the  two  lateral  halves  of  the  hard 
palate,  by  which  the  primitive  oral  cavity  is  divided  into  the 
mouth  proper  and  the  nasal  chambers,  the  tongue  projects 
upward  between  the  palate-shelves,  almost  completely  filling 
the  primitive  mouth.  As  the  palate-shelves  approach  each 
other,  however,  the  tongue  gradually  recedes  to  its  subsequent 
normal  position. 

THE  DEVELOPMENT  OF  THE   NOSE. 

The  nose  being  an  organ  of  special  sense,  its  development 
is  described  in  connection  with  that  of  the  other  special-sense 
organs  in  Chapter  XVL  Owing,  however,  to  its  important 
relation  to  the  other  parts  of  the  face,  it  is  desirable  to  refer 
to  its  evolution  in  this  connection.  For  a  more  detailed 
account,  the  reader  is  referred  to  Chapter  XVI. 

The  first  indication  of  the  organ  of  smell  is  in  the  form 
of  the  two  patches  of  thickened  ectoderm,  the  nasal  areas  or 
olfactory  plates,  which  appear  on  the  headward  side  of  the 
oral  fossa  in  the  third  week  of  development.  At  the  end  of 
the  fourth  week  the  areas  are  depressed  and  constitute  the 
nasal  pits  (Fig.  56,  A).  The  nasofrontal  process,  a  mass  of 
thickened  mesodermic  tissue,  lies  between  them.    During  the 


134  TEXT-BOOK  OF  EMBRYOLOGY. 

fifth  week  the  lateral  edges  of  this  process  become  thick  and 
rounded,  forming  the  two  globular  processes,  while  growing 
outward  and  downward  from  the  sides  of  its  base  are  the  two 
lateral  nasal  or  lateral  frontal  processes.  Thus  the  nasal  pits, 
which  correspond  with  the  position  of  the  future  anterior 
nares,  become  bordered  on  the  mesial  side  by  the  globular 
processes  and  on  the  outer  side  by  the  lateral  nasal  processes. 
Below,  the  pits  are  continuous  with  the  oral  fossa.  Owing 
to  the  continued  growth  of  these  masses  the  pits  gradually 
become  deeper.  The  lateral  nasal  process  is  separated  ex- 
ternally from  the  maxillary  process  of  the  first  visceral  arch 
by  a  groove,  the  naso-optic  furrow.  The  lower  extremities  of 
the  maxillary  and  lateral  nasal  processes  soon  unite  Avith  each 
other  and  advance  toward  the  median  line  below  the  nasal 
pit.  In  the  latter  part  of  the  sixth  week  they  unite  with 
the  nasofrontal  process  and  thus  separate  the  nasal  pits  from 
the  oral  fossa  and  furnish  the  basis  of  the  upper  lip.  The 
nasal  pits  are  now  the  anterior  nares,  and  the  nose  is  repre- 
sented by  the  irregular  masses  of  tissue  surrounding  them. 
While  the  orifices  of  the  nares  are  separated  from  the  orifice 
of  the  primitive  oral  cavity,  their  deeper  parts  are  continuous 
with  the  latter,  there  being  as  yet  no  hard  or  soft  palate. 

In  the  eighth  week  the  nose  first  acquires  definite  form, 
owing  to  the  continued  growth  of  the  masses  of  tissue  re- 
ferred to  above.  The  nasofrontal  process  forms  the  bridge 
of  the  nose  with  the  nasal  septum,  and  also  the  intermaxillary 
part  of  the  superior  maxillse  and  the  connective-tissue  parts 
of  the  upper  lip.  The  lateral  frontal  process  becomes  the 
ala  of  the  nose.  The  nose  is  still  very  broad  and  flat  in  the 
third  month,  after  which  time  it  gradually  assumes  its  char- 
acteristic form. 


CHAPTER   X. 
THE   DEVELOPMENT  OF  THE   VASCULAR  SYSTEM. 

The  vascular  system,  including  the  blood,  the  heart,  and 
the  blood-vessels,  begins  its  development  very  early  in  em- 
bryonic life. 

While  the  heart  is  formed  within  the  body  of  the  embryo, 
the  blood  and  the  earliest  blood-vessels  have  their  origin  in 
an  extra-embryonic  structure,  the  yolk-sac.  It  is  note- 
worthy that  all  parts  of  the  vascular  system  proceed  from 
mesodermic  tissue,  the  heart  and  the  vessels  originating  from 
clefts  within  this  structure,  and  being  lined,  therefore,  with 
endothelial  cells. 

In  correspondence  with  the  varying  relations  which  the 
embryo  sustains  toward  the  fetal  appendages  at  different 
times,  its  circulatory  system  is  distinguished  successively 
by  certain  special  features.  Thus,  during  the  activity  of 
the  yolk-sac  as  an  organ  of  nutrition,  the  vitelline  circulation 
is  present;  following  and  supplanting  this  is  the  allantoic 
circulation,  which  latter,  in  turn,  gives  place  to,  or,  in  fact, 
becomes  the  placental  system  of  vessels. 

THE  VITELLINE  CIRCULATION  AND  THE  ORIGIN 
OF  THE  BLOOD. 

The  seat  of  the  first  formation  of  the  blood-vessels  and 
of  the  blood  is  the  wall  of  the  yolk-sac,  entirely  outside  of 
the  body  of  the  embryo.  The  wall  of  the  yolk-sac,  the 
reader  may  be  reminded,  consists  of  the  extra-embryonic 
splanchnopleure  covered  with  a  part  of  the  somatopleure. 
The  mesodermic  layer  of  the  sac  exhibits — at  the  end  of  the 
first  day  in  the  chick — a  network  made  up  of  cords  of  cells. 
Interspersed  throughout  this  network,  as  part  of  it,  are 
groups  of  mesodermic  cells  known  as  the  blood-islands  of 

135 


136  TEXT-BOOK  OF  EMBBYOLOOY. 

Pander.  Soon"  the  cords  composing  the  network  become 
hollowed  out  so  as  to  constitute  an  anastomosing  reticulum 
of  vessels,  the  cells  which  are  to  compose  the  walls  of  these 
vessels  meanwhile  acquiring  the  endothelial  type.  The  re- 
maining component  cells  of  the  blood-islands  coincidentally 
undergo  partial  disintegration  and  liquefaction,  by  which 
their  nuclei  are  set  free.  These  nuclei,  surrounded  by  a 
very  small  and  inconspicuous  envelope  of  protoplasm,  are 
the  primitive  red  blood-corpuscles  and  are,  during  the  first 
month,  the  only  corpuscular  elements  that  the  fetal  blood 
contains.  They  differ  essentially  from  the  adult  red  cor- 
puscles in  the  fact  of  their  being  nucleated.  Their  shape  is 
at  first  spherical  or  spheroidal.  Subsequently  they  entirely 
disappear.  Other  cells  are  added  to  the  blood  during  the 
second  month  and  afterward.  Without  discussing  the  un- 
settled question  of  the  origin  of  the  adult  blood-cells — the 
red  and  the  white  corpuscles  and  the  blood-plaques — it  may 
be  pointed  out  in  passing  that  the  liver  is  a  very  important 
source  of  the  red  corpuscles  during  fetal  life. 

Limiting  the  first  network  of  vessels  on  the  surface  of  the 
yolk-sac  is  a  circular  vessel,  the  sinus  terminalis  (Plate  YL). 
Since  the  yolk-sac  is  relatively  so  large  that  the  body  of  the 
embryo  appears  to  rest  upon  it,  and  since  the  surrounding 
somatopleure  is  translucent,  a  surface  view  of  the  ovum  at 
this  stage  shows  a  vascular  zone  encircling  the  embryonic 
area  and  the  later  body  of  the  embryo.  This  zone  is  the 
area  vasculosa,  or  vascular  area,  the  seat  of  the  earliest  for- 
mation of  blood  and  of  blood-vessels  of  the  embryo. 

The  vessels,  by  a  process  of  budding,  grow  from  the  vas- 
cular area  along  the  vitelline  duct  into  the  body  of  the 
embryo.  Here  they  take  their  course  toward  the  primitive 
heart,  which  has  meanwhile  been  developing.  From  the 
anterior  and  posterior  and  lateral  limits  of  the  vascular  area 
— using  these  terms  with  reference  to  the  axis  of  the  em- 
bryonic body — four  pairs  of  vitelline  veins  converge  toward 
the  vitelline  duct  and  unite  to  form  the  two  vitelline  or 
omphalomesenteric  veins.  These  veins,  after  entering  the 
body  of  the  embryo,  pass  headward  along  the  wall  of  the 


PLATE   VI. 


Vascular  area  of  eleven-day  rabbit  embryo  (E.  von  Beneden  and  Julin).   The  capillaries 
are  not  shown ;  the  terminal  sinus  is  seen  to  be  arterial. 


THE   VITELLINE  CIRCULATION.  137 

intestinal  tube  and  empty  into  the  lower  or  caudal  end  of 
the  primitive  heart.  The  trunks  which  are  to  constitute  the 
vitelline  arteries,  after  entering  the  body  with  the  vitelline 
duct,  pass  upward  along  the  dorsal  body-wall,  within  the 
dorsal  mesentery,  to  become  continuous  with  large  arterial 
trunks  that  have  proceeded  from  the  primitive  heart. 

The  large  trunks  referred  to  are  the  visceral-arch  vessels, 
which  unite  to  form  the  primitive  aortee.  The  visceral-arch 
vessels  (see  Fig.  49)  are  a  series  of  five  pairs  of  arteries  that 
arise  by  a  common  stem,  the  truncus  arteriosus,  from  the 
upper  end  of  the  primitive  heart.  They  pass  along  the 
respective  visceral  arches  toward  the  dorsal  surface  of  the 
body  where  all  the  vessels  of  one  side  unite  into  a  common 
trunk,  the  primitive  aorta.  The  two  primitive  aortse,  pass- 
ing caudalward  in  the  dorsal  mesentery,  give  oflT,  as  their 
largest  branches,  the  two  omphalomesenteric  or  vitelline 
arteries  above  referred  to.  The  development  and  the  re- 
gression of  the  visceral-arch  vessels  correspond  with  the 
growth  and  the  decadence  respectively  of  the  visceral  arches. 
Not  all  the  vessels  are  present  in  a  fully-developed  condition 
at  any  one  time,  the  first  pair  having  begun  to  atrophy  before 
the  fifth  pair  makes  its  appearance.  The  metamorphosis 
into  certain  adult  vessels  of  such  of  them  as  persist  will  be 
considered  in  a  later  section. 

This  system  of  vessels  constitutes  the  vitelline  circulation, 
the  manifest  function  of  which  is  to  convey  nutritive  mate- 
rial from  the  yolk-sac  to  the  embryo.  While  the  vitelline 
circulation  is  of  great  importance  in  any  ovum  provided 
with  abundant  nutritive  yolk,  such  as  that  of  the  bird,  it  is 
of  comparatively  slight  consequence  in  man  and  the  other 
higher  mammals,  and  it  must  be  regarded  as  a  vestige  of  the 
avian  or  reptilian  ancestry  of  the  mammalian  ovum,  or,  at 
least,  as  a  reminder  that  the  mammalian  ovum  was  originally 
provided  with  an  abundant  yolk.  It  must  be  borne  in  mind, 
however,  that  the  mammalian  blastodermic  vesicle  imbibes 
from  the  walls  of  the  uterus  a  richly  nutritive  albuminous 
fluid,  which  may  be  taken  up  later  and  carried  to  the  em- 
bryo by  the  vitelline  circulation.     This  system  of  yolk-sac 


138 


TEXT-BOOK  OF  EMBRYOLOGY. 


vessels  disappears  with  the  regression  and  disappearance  of 
the  yolk-sac — in  the  human  embryo  at  about  the  fifth  week. 

To  render  the  comprehension  of  the  later  phases  of  the 
vascular  system  more  simple,  their  consideration  is  deferred 
until  the  development  of  the  heart  shall  have  been  described. 

THE  DEVELOPMENT  OF  THE  HEART, 

The  heart,  when  studied  in  the  lower-type  animals,  is 
seen  to  be  morphologically  a  dilated  and  specialized  part  of 
a  vascular  trunk  embedded  in  the  ventral  mesentery.  In 
man,  the  first  fundament  of  the  heart  appears  at  a  very  early 
period — namely,  before  the  splanchnopleure  has  folded  in  to 


ParUtal  Mesoderm. 


f/eart-Pute 


■PLcuroJincardial  Cauity 

_ ^earT-Piare  orOuXa-M^c 

\/iscera.cMesoUer>n.  '  ^Mesocardiiun.  Atertus 

Fig.  61.— Schematic  cross-section  of  rabbit-embryo  to  show  development  of 
heart ;  A,  embryonic  area  with  the  germ-membranes  still  spread  out ;  B,  more 
advanced  stage,  the  splanchnopleure  partly  folded  in  ;  C,  splanchnopleure  folded 
in  to  form  gut-tract,  the  two  heart-tubes  fused  into  one  (after  Strahl). 

form  the  gut^tract,  or,  in  other  words,  before  the  end  of  the 
second  week.  This  fundament,  in  all  higher  vertebrates,  is 
bilateral,  having  the  form  of  two  tubes  produced  by  vacuola- 
tion  of  the  splanchnic  mesoderm  and  lying  widely  separated, 
one  in  each  half  of  the  still  spread  out  splanchnopleure 
(Fig.  61,  A).     A  transverse  section  through  the  future  neck- 


THE  DEVELOPMENT   OF  THE  HEART. 


139 


region  of  a  sheep-  or  rabbit-embryo  shows  the  tubes  cut 
across,  since  their  long  axes  are  parallel  with  that  of  the 
body  (Fig.  62).  With  the  folding  in  of  the  splanchnopleure 
and  the  union  of  the  edges  of  its  folds,  the  tubes  are  carried 
toward  each  other,  and  subsequently,  by  the  disappearance 
of  the  tissue  intervening  between  them,  their  cavities  become 
one  (Fig.  61,  B  and   (7).     After  the  formation  of  the  gut- 


Amnion. 


MesoderTn. 


Visceral 
7nesoder*n. 


Pleuropericar-         Pericardial 
dial  cavity.  plates. 


Extension 
of  cceloiii. 


Fig.  62.— Transverse  section  of  a  sixteen-and-a-half-day  sheep-embryo  (Bonnet). 

tract,  therefore,  and  the  simultaneous  appearance  of  the 
ventral  body-wall,  the  heart-fundament  is  a  single  straight 
mesodermic  tube,  situated  in  the  pharyngeal  region,  in  close 
relation  with  the  ventral  wall  of  the  body,  between  the  latter 
and  the  fore- gut.  Reference  to  Fig.  61,  C,  will  show  that 
the  heart-tube  is  separated  from  the  body-cavity  (or  coelom) 
on  each  side  by  a  layer  of  the  mesoderm,  and  that  these  two 
layers  connect  the  heart  dorsally  with  the  gut-tract  and 
'  ventrally  with  the  body-wall,  forming  respectively  the  meso- 
cardium  anterius  and  the  mesocardium  posterius.  These  folds 
temporarily  divide  the  upper  portion  of  the  btKly-cavity  into 
two  lateral  parts. 


140 


TEXT-BOOK  OF  EMBRYOLOGY. 


The  disappearance  of  the  stratum  of  mesoderm  imme- 
diately surrounding  the  heart-tube  and  the  differentiation 
of  the  tissue  limiting  peripherally  the  space  thus  formed, 
results  in  the  production  of  a  second  larger  tube  enclosing 
the  first.  The  cells  of  the  outer  tube  become  specialized 
into  muscle-cells,  which  are  to  constitute  the  future  heart- 
muscle,  while  those  of  the  inner  cylinder  flatten  and  assume 
the  endothelioid  type  to  become  the  endocardium.  The  growth 
of  centrally  projecting  processes  from  the  muscular  wall  and 
the  outpocketing  of  the  endothelial  tube  to  cover  these 
processes  and  line  the  spaces  enclosed  by  them  foreshadow 
the  spongy  character  of  the  inner  surface  of  the  adult  heart, 
with  its  columnae  carneae  and  musculi  pectinati.  It  is  signifi- 
cant, as  showing  the  contractility  of  undifferentiated  proto- 


FiG.  63.— Diagrams  illustrating  arrangement  of  primitive  heart  and  aortic 
arches  (modified  from  Allen  Thomson) :  1,  vitelline  veins  returning  blood  from 
vascular  area;  2,  venous  segment  of  heart-tube;  3,  primitive  ventricle;  4,  truncus 
arteriosus ;  5,  5,  upper  and  lower  primitive  aortEe ;  5',  5',  continuation  of  double 
aortae  as  vessels  to  caudal  pole  of  embryo ;  6,  vitelline  arteries  returning  blood  to 
vascular  area. 

plasmic  cells,  that  the  heart  begins  to  pulsate  even  before 
the  appearance  of  any  muscular  tissue  in  its  walls. 

The  upper  end  of  the  heart-tube  tapers  away  into  the 
tnincus  arteriosus  (Fig.  63,  4),  a  vessel  which  bifurcates  into 
the  first  pair  of  visceral-arch  vessels,  while  its  lower  ex- 


THE  DEVELOPMENT  OF  THE  HEART. 


141 


tremity  receives  the  vitelline  veins  above  referred  to.  Ex- 
cessive growth  in  length,  each  end  of  the  tube  being  more 
or  less  fixed  in  position,  necessitates  flexion  or  folding,  the 
form  which  the  heart-tube  assumes  in  consequence  being  that 
of  the  letter  S  placed  obliquely  (Fig.  64,  A).     The  venous 


^L^ 


Fig.  64.— a,  heart  of  human  embryo  of  2.15  mm.  (His) :  a,  truncus  arteriosus; 
6,  primitive  ventricle;  c,  venous  segment.  B,  lieart  of  human  embryo  of  about 
3  mm.  (His) :  a,  truncus  arteriosus  ;  6,  venous  segment  (behind) ;  c,  primitive  ven- 
tricle (in  front). 

limb  of  the  S  lies  caudad  and  toward  the  left,  the  arterial  seg- 
ment being  directed  headward  and  toward  the  right,  so  that 
the  two  lie  almost  in  the  same  coronal  plane.  These  rela- 
tions are  soon  altered  by  such  a  rotation  around  a  longitudinal 
axis  that  the  venous  part  of  the  heart  comes  to  lie  nearei* 
the  dorsal  wall  of  the  body,  with  the  arterial  portion  ventral 
to  it,  both  being  brought  at  the  same  time  into  practically 
one  transverse  plane  by  the  headward  migration  of  the 
venous,  and  the  tailward  migration  of  the  arterial,  moiety. 
At  this  time  the  heart  is  relatively  so  large,  and  the  ventral 
body-wall  covering  it  so  thin,  that  the  organ  appears  as  if 
situated  outside  of  the  embryo's  body  (Fig.  51,  p.  104). 

Simultaneously  with  these  alterations  in  position,  the  ar- 
terial part  of  the  heart  is  being  marked  off  from  the  venous 
segment  by  a  transverse  constriction,  the  former  becoming  the 
ventricle,  the  latter  the  auricle  or  atrium  (Fig.  64,  A).  The 
narrow  communication  between  the  two  is  the  auricular  or 
atrioventricular    canal,    which    soon    acquires   the   primitive 


142 


TEXT-BOOK  OF  EMBRYOLOGY. 


atrioventricular  valves.  The  truncus  arteriosus  becomes  de- 
limited from  the  ventricle  by  a  circular  constriction,  the 
fretum  Halleri,  the  proximal  part  of  the  truncus  arteriosus 
dilating  somewhat  to  constitute  the  bulbus  arteriosus.  The 
truncus  arteriosus  divides  into  the  visceral-arch  vessels,  as 
pointed  out  in  the  last  section. 

The  Metamorphosis  of  the  Single  into  the  Double 
Heart. — The  heart  with  but  one  ventricle  and  one  auricle  or 
atrium  is  found  not  only  during  the  early  periods  of  develop- 
ment in  all  air-breathing  vertebrates,  but  is  the  permanent 
condition  in  fishes.  In  the  development  of  the  individual, 
as  in  the  evolution  of  the  higher  vertebrate  type,  the  appear- 
ance of  the  lungs,  which  replace  the  branchiae  of  fishes  as  an 
aerating  apparatus,  is  accompanied  by  a  division  of  the  heart 


Fig.  65. — A,  heart  of  human  embryo  of  about  4.3  mm.  (His) :  a,  atrium ;  6,  por- 
tion of  atrium  corresponding  with  auricular  appendage ;  c,  truncus  arteriosus  ;  d, 
auricular  canal ;  e,  primitive  ventricle.  B,  heart  of  human  embryo  of  about  the 
fifth  week  (His) :  a,  left  auricle ;  b,  right  auricle ;  c,  truncus  arteriosus ;  d,  inter- 
ventricular groove ;  e,  right  ventricle ;  /,  left  ventricle. 


into  right  and  left  halves  for  the  pulmonary  and  the  general 
systemic  circulation  respectively. 

The  division  of  the  human  atrium  begins  in  the  fourth  iveck 
with  the  growth  of  a  perpendicular  ridge  from  its  dorsal  and 
cephalic  walls  (Fig.  66,  B),  which  extends  toward  the  cavity 
and  ultimately  divides  it  into  right  and  left  auricles  (Fig.  65). 
The  atrioventricular  canal,  with  its  anterior  and  posterior 


THE  DEVELOPMENT  OF  THE  HEART. 


143 


ridges  or  valves,  shares  in  this  partition,  becorai-ng-  thereby 
the  right  and  the  left  auriculoventricular  orifices.  The 
separation  of  the  atrium,  however,  is  not  complete,  since 
there  remains  in  the  septum  an  aperture,  the  foramen  ovale, 
which  persists  until  birth  or  shortly  after/ 

The  division  of  the  ventricle,  which  follows  that  of  the 
auricle  and  which  is  completed  by  the  seventh  week,  is  first 
indicated  by  a  vertical  groove,  the  sulcus  interventricularis, 
seen  on  both  the  dorsal  and  the  ventral  surface  (Fig.  65). 


Fig.  66. — A,  section  of  heart  of  human  embryo  of  10  mm.  (His) :  a,  septum 
spurium;  6,  interauricular  septum;  c,  mouth  of  sinus  reuniens ;  d,  right  auricle; 
e,  left  auricle  ;  /,  auricular  canal ;  g,  right  ventricle  ;  h,  interventricular  septum ; 
i,  left  ventricle.  B,  section  of  heart  of  human  embryo  of  about  the  fifth  week 
(His):  a,  septum  spurium:  b,  auricular  septum;  c,  opening  of  sinus  reuniens 
(leader  passes  through  foramen  ovale) ;  d,  right  atrium ;  e,  left  atrium ;  /,  septum 
intermedium ;  g,  right  ventricle  ;  h,  ventricular  septum ;  i,  left  ventricle. 

From  the  internal  wrface,  corresponding  to  the  position  of 
the  sulcus,  a  median  centrally  projecting  ridge  appears  and 
develops  into  a  septum,  thus  producing  the  right  and  the  left 
ventricles  (Figs.  6Q  and  67). 

The  truncus  arteriosus,  after  having  become  somewhat  flat- 
tened, by  the  growth  of  a  vertical  septum,  or  partition  (Fig. 

'  Occasionally  the  foramen  ovale  remains  patulous  for  several  weeks  or 
months  after  birth  or  even  throughout  life.  As  this  condition  allows  the 
venous  blood  to  mingle  with  the  arterial,  the  surface  of  the  body  is  bluish 
or  cj'anotic,  and  a  child  thus  affected  is  said  to  be  a  "  blue  baby." 


144  TEXT-BOOK  OF  EMBRYOLOGY. 

67,  s),  is  divided  into  the  aorta  and  the  pulmonary  artery. 
Though  the  three  septa  referred  to  develop  independently  of 
each  other  there  is  such  correspondence  between  them,  as  to 
position,  that  the  effect  is  as  if  they  constituted  one  contin- 
uous structure. 

Before  the  division  of  the  atrium  into  the  auricles,  its 
walls  pouch  out  on  each  side  to  form  the  auricular  appen- 
dages, one  of  which  belongs  to  each  future  auricle  (Fig.  65). 
While  it  is  still  a  straight  tube,  the  heart  receives  at  its 
venous  extremity  the  two  vitelline  veins.  Subsequently  this 
particular  part  of  the  atrium  is  distinguished  as  the  sinus 
venosus  or  sinus  reuniens,  this  being  a  short  thick  trunk  into 
which  empty,  in  addition  to  the  vitelline  veins,  the  ducts  of 
Cuvier  and  the  umbilical  veins.  The  mouth  of  the  sinus 
venosus  is  guarded  by  a  valve  composed  of  two  leajflets.  In 
the  division  of  the  atrium  the  sinus  venosus  falls  to  the  right 
auricle,  while  emptying  into  the  left  auricle  is  the  single  pul- 
monary vein,  which  is  formed  by  the  union  of  the  four  pul- 
monary veins.  Still  later,  the  sinus  venosus  is  merged  into 
the  wall  of  the  right  auricle,  and  hence  the  venous  trunks 
above  mentioned  empty  by  separate  orifices  into  its  cavity. 

The  left  leaflet  of  the  valve  at  the  mouth  of  the  sinus 
venosus  becomes  atrophic ;  the  right  divides  into  two  parts, 
one  of  which  becomes  the  Eustachian  valve  at  the  orifice  of 
the  inferior  vena  cava,  while  the  other  forms  the  valve  of 
Thebesius,  or  the  coronary  valve,  at  the  opening  of  the  coro- 
nary sinus  (the  latter  being  the  persistent  lower  end  of  the 
left  duct  of  Cuvier).  The  Eustachian  valve  serves  to  direct 
the  blood  from  the  inferior  cava  through  the  foramen  ovale 
so  long  as  that  aperture  is  present.  The  single  pulmonary 
vein  is  in  like  manner  incorporated  in  the  wall  of  the  left 
auricle,  the  four  pulmonary  veins  in  consequence  acquiring 
separate  openings  into  that  cavity. 

The  Valves  of  the  Heart. — Before  the  division  of  the 
atrium  and  the  ventricle  into  right  and  left  halves,  the  atrio- 
ventricular canal  has  the  form  of  a  transverse  fissure,  each 
lip  of  which  is  thickened  into  a  ridge  (Fig.  67,  A).  These 
ridges   or   endocardial    cushions   are    the    primitive   valves. 


THE  DEVELOPMENT  OF  THE  HEART. 


145 


When  the  atrial  partition  grows  down  and  the  ventricular 
septum  grows  up,  their  free  edges  meet  and  unite  with  the 
ridges,  each  ridge  being  thereby  divided,  on  its  atrial  surface 
by  the  atrial  or  interauricular  septum,  and  on  its  ventricular 
aspect  by  the  ventricular  septum,  into  a  right  and  a  left  half 
(Fig.  67,  B).     Since  the  ridges,  at  their  points  of  union  with 


Fig.  67.— Two  diagrams  (after  Born)  to  elucidate  the  changes  in  the  mutual 
relations  of  the  interventricular  orifice  and  the  ostium  interventriculare  as  well 
as  the  division  of  the  ventricle  and  large  arteries.  The  ventricles  are  imagined  to 
have  been  divided  into  halves ;  one  looks  into  the  posterior  (dorsal)  halves,  in 
which,  moreover,  the  cardiac  trabeculee,  etc.,  have  been  omitted  for  the  sake  of 
simplifying  the  view.  A,  heart  of  an  embryo  rabbit,  in  which  the  head  is  8.5-5.8 
mm.  long.  The  ventricle  is  divided  by  the  ventricular  partition  (ks)  into  a  left 
and  a  right  half  as  far  as  the  interventricular  orifice  [Oi).  The  right  end  of  the 
foramen  atrioventriculare  commune  (F.  av.  c)  extends  into  the  right  ventricle ;  the 
endocardial  cushions  (o.  ek,  u.  ek)  are  developed.  B,  heart  of  an  embryo  rabbit, 
head  7.5  mm.  long.  The  endocardial  cushions  (p.  ek,  u.  ek)  of  the  foramen  atrioven- 
triculare commune  are  fused,  and  thereby  the  foramen  atrioventriculare  commune 
is  now  separated  into  a  foramen  atrioventriculare  dextrum  (F.  av.  d)  and  sinistrum 
{F.av.s).  The  ventricular  partition  (ks)  has  likewise  fused  with  the  endocardial 
cushions,  and  has  grown  forward  as  far  as  the  partition  (s)  of  the  truncus  arterio- 
sus. By  the  closure  of  the  remnant  of  the  ostium  interventriculare  [Oi)  the  sep- 
tum membranaceum  is  formed;  rk,  right,  Ik,  left  ventricle;  ks,  ventricular  parti- 
tion; Pm,  arteria  pulmonalis ;  ^o,  aorta;  s,  partition  of  the  truncus  arteriosus ;  Oi, 
ostium  interventriculare  ;  F.  av.  c,  foramen  atrioventriculare  commune  ;  F.  av.  d 
and  F.  av.  s,  foramen  atrioventriculare  dextrum  and  sinistrum ;  o.  ek,  u.  ek,  upper 
and  lower  endothelial  or  endocardial  cushions. 


the  septa,  fuse  likewise  with  each  other,  the  original  orifice 
is  bisected  into  the  right  and  left  auriculoventricular  aper- 
tures, the  only  valves  of  which  are  the  ridges  or  cushions  in 
question. 

To  trace  the  further  development  of  the    fully    formed 
valves,  it  will  be  necessary  to  consider  the  changes  which 

10 


146 


TEXT-BOOK  OF  EMBRYOLOGY. 


now  take  place  in  the  Avails  of  the  heart.  It  has  been  seen 
that  the  inner  surface  of  the  heart  acquires  a  spongy  or 
trabecular  structure  at  a  very  early  stage  by  the  inward  pro- 
jection of  muscular  processes  from  the  outer  tube  and  the 
pouching  out  of  the  inner  endothelial  tube  to  cover  these. 
The  wall  of  the  ventricle  in  consequence  is  relatively  very 
thick  and  is  made  up  largely  of  a  network  of  fleshy  columns, 
the  spaces  of  which  network  are  lined  with  the  endocardium 
(Fig.  68,  A).     While  the  outer  stratum  of  the  ventricular 


Fig.  68. — Diagrammatic  representation  of  the  formation  of  the  atrioventricular 
valves:  A,  earli  r,  £,  later  condition  (after  Gegenbaur) ;  ?)iA-,  membranous  valve; 
mfc',  the  primitive  part  of  the  same ;  cht,  chordae  tendinese ;  v,  cavity  of  the  ventri- 
cle ;  6,  trabecular  network  of  cardiac  musculature ;  pm,  papillary  muscles  ;  tc,  ira- 
beculse  carnese. 


wall  now  becomes  more  compact  by  the  thickening  of  the 
trabeculse — and,  to  some  extent,  by  their  coalescence — the 
trabeculse  in  the  vicinity  of  the  atrioventricular  valves  di- 
minish in  thickness-  and  lose  their  muscular  character,  being 
replaced  by  thin  connective-tissue  cords  (Fig.  68,  B).  That 
part  of  the  ventricular  wall  which  surrounds  the  atrioven- 
tricular orifice  and  to  which  the  endocardial  cushions  or 
primitive  valves  are  attached,  likewise  becomes  deprived 
of  muscle-cells,  the  remaining  connective  tissue  assuming 
the  form  of  thin  plates.  These  plates,  with  the  former 
endocardial  cushions  attached  to  their  edges,  constitute  the 
permanent  auriculoventricular  valve-leaflets.  The  strands  of 
connective  tissue  mentioned  above  as  remaining  after  the 
degeneration  of  certain  of  the  muscle-trabeculse  are  the 
chordae  tendinese  of  the  adult  heart.  Attached  at  one  end 
to    the    valve-leaflets,  their   other   extremity    is   continuous 


THE  DEVELOPMENT  OF  THE  HEART. 


147 


with  trabeculse  that  have  remained  muscular,  the  adult  mus- 
culi  papillares. 

The  semilunar  valves  of  the  aorta  and  pulmonary  artery 
appear  when  the  truncus  arteriosus  divides  to  form  those 
vessels.  The  orifice  of  the  truncus  arteriosus  is  provided 
with  a  valve  having  four  leaflets  (Fig.  69,  A).  By  the  di- 
vision of  this  vessel  into  the  pulmonary  artery  and  the  aorta 
(Fig.  69,  B  and  C),  the  lateral  leaflets  are  bisected,  the  ante- 
rior half  of  each,  with  the  anterior  leaflet,  going  to  the  ante- 
rior vessel — the  pulmonary  artery — while  each  posterior  or 
dorsal  half,  with  the  dorsal  leaflet,  falls  within  the  orifice  of 
the  aorta.  The  resulting  disposition  of  the  segments  of  the 
aortic  and  pulmonary  valves  is  such  that,  in  the  aorta,  two 
leaflets  are  situated  anteriorly  and  one  posteriorly,  while  in 
the  case  of  the  pulmonary  artery  these  conditions  are  reversed 
(Fig.  69,  C).  In  the  fully  developed  heart,  however,  it  is 
found  that  the  aorta  has  two  posterior  leaflets  and  one  ante- 
rior, and  that  the  pulmonary  artery  presents  one  posterior 


Jortc 


j4orta. 


/^ulfnonaj-y  ccttery 


Fig.  69.— Scheme  showing  division  of  truncus  arteriosus  and  its  valve-leaflets 
into  aorta  and  pulmonary  artery  with  their  leaflets.  The  division  begins  in  B,  the 
lateral  leaflets  dividing  respectively  into  a,  e,  and  c,  /.  Rotation  from  right  to  left 
shown  in  D. 


and  two  anterior  segments.  In  the  division  of  the  truncus 
arteriosus,  the  anterior  half,  or  the  pulmonary  artery,  falls 
to  the  right  ventricle,  and  the  posterior  trunk,  the  aorta,  to 
the  left  ventricle,  the  two  ventricles  lying  side  by  side.  In 
order,  therefore,  that  the  ventricles  may  acquire  the  relative 
positions  which  they  hold  in  the  adult  there  must  be  such  a 
rotation  that  the  left  ventricle  comes  to  lie  behind  the  right. 


148  TEXT-BOOK  OF  EMBRYOLOGY. 

This  rotation  of  the  heart  from  right  to  left  necessarily  alters 
the  relation  of  the  pulmonary  artery,  causing  it  to  lie  not 
directly  in  front  of  the  aorta,  but  in  front  and  to  the  left. 
If  one  conceives  of  a  rotation  of  the  two  vessels  from  right 
to  left  through  an  arc  of  60  degrees  around  a  vertical  axis, 
the  altered  relation  of  the  pulmonary  and  aortic  leaflets  be- 
comes at  once  intelligible  (Fig.  69,  C  and  D). 

THE  ALLANTOIC  AND  THE  PLACENTAL  CIRCULATION. 

The  development  of  the  allantois  and  its  accompanying 
system  of  blood-vessels  is  simultaneous  with  the  decline  of 
the  yolk-sac  and  the  vitelline  circulation.  Since  the  allan- 
tois is  an  evagination  from  the  gut-tract  (see  p.  80),  it  is  a 
splanchopleuric  sac,  its  walls  consisting  therefore  of  an  ento- 
dermic  and  a  mesodermic  layer.  Blood-vessels  develop 
Avithin  the  mesodermic  stratum  as  extensions  or  branches 
of  previously  existing  intra-embryonic  trunks.  These  ves- 
sels are  the  allantoic  arteries  and  veins.  The  two  allantoic 
arteries  are  branches  of  the  primitive  aorta  and  leave  the 
body  of  the  fetus,  in  company  with  the  neck  of  the  allantois, 
at  the  umbilicus.  Having  reached  the  peripheral  part  of  the 
allantois,  they  break  up  into  a  capillary  plexus,  the  extension 
of  which  into  the  villous  processes  of  the  false  amnion  com- 
pletes the  union  of  that  structure  with  the  allantois  to  form 
the  true  chorion  (Plate  III.). 

The  two  allantoic  veins  develop  'pari  passu  with  the  arteries 
and  convey  the  blood  from  the  chorion  to  the  fetus.  En- 
tering the  body  of  the  fetus  through  the  still  large  um- 
bilical aperture,  they  find  their  way  along  the  intestinal  tube 
to  the  septum  transversum — which  structure  may  be  regarded 
as  the  primitive  diaphragm — to  the  region  of  the  heart, 
where  they  open  into  the  ducts  of  Cuvier.  Each  duct  of 
Cuvier  (Fig.  72,  A)  is  formed  by  the  union  of  the  primitive 
jugular  vein  with  the  cardinal  vein  of  its  own  side,  the  car- 
dinal and  the  jugular  veins  returning  the  blood  respectively 
from  the  lower  and  upper  parts  of  the  trunk.  This  system 
of  blood-vessels  constitutes  the  allantoic  circulation ;  it  is  of 
great  importance  in  any  ovum  that  is  developed  outside  of 


THE  FETAL  ARTERIAL  SYSTEM.  149 

the  body  of  the  mother,  as  in  the  case  of  birds,  reptiles,  and 
fishes,  in  which  classes  the  allantois  is  the  organ  of  nutrition 
from  the  time  that  the  yolk-sac  ceases  to  perform  that  func- 
tion until  development  is  complete.  In  man,  however,  as  in 
all  other  mammals  except  the  monotremes  and  marsupials, 
the  allantoic  circulation  may  be  looked  upon  as,  in  a  measure, 
rudimentary,  since  it  serves  to  convey  nutriment  from  the 
chorion  to  the  fetus  only  until  the  formation  of  the  placenta. 
The  placental  system  of  blood-vessels,  appearing 
in  the  third  month  with  the  development  of  the  placenta, 
includes  the  principal  trunks  of  the  former  allantoic  system, 
the  allantoic  arteries  and  veins  having  become  the  umbilical 
vessels.  The  two  umbilical  arteries  convey  impure  blood 
from  the  fetus  to  the  placenta,  where  it  circulates  through 
the  capillaries  of  that  organ  and  receives  oxygen  and  nutriment 
from  the  blood  of  the  mother.  As  before  stated,  there  is  no 
intermingling  of  the  fetal  and  the  maternal  blood,  the  two 
currents  being  separated  by  the  very  thin  walls  of  the  capil- 
laries, through  which  osmosis  occurs.  The  purified  blood 
returns  to  the  fetus  through  the  umbilical  veins  and  reaches 
the  right  auricle  through  the  inferior  vena  cava,  a  portion  of 
it  having  passed  through  the  liver.  The  two  umbilical  veins 
which  are  present  for  a  time  fuse  subsequently  to  form  a 
single  vein.  The  complicated  details  of  the  arterial  and  the 
venous  trunks,  and  the  relation  of  the  latter  to  the  develop- 
ment of  the  liver  and  its  special  system  of  vessels,  may  be 
advantageously  considered  in  separate  sections. 

THE   FETAL  ARTERIAL  SYSTEM. 

The  truncus  arteriosus,  the  large  artery  which  arises  from 
the,  as  yet,  undivided  ventricle  of  the  heart,  bifurcates  into 
two  trunks,  the  first  pair  of  visceral-arch  vessels  (Fig.  70,  4). 
These  first  visceral-arch  vessels,  also  sometimes  called  the 
first  aortic  arches,  run  from  the  ventral  surface  of  the  body 
along  the  first  visceral  arches,  toward  the  dorsum,  where 
they  curve  downward  and  pass  caudalward,  one  on  each  side 
of  the  median  line,  in  front  of  the  primitive  vertebral  column. 
Very  soon  there  arise  from  the  truncus  arteriosus  below  the 


150 


TEXT-BOOK  OF  EMBRYOLOGY. 


point  of  origin  of  the  first  vessels,  four  additional  pairs  of 
visceral-arch  vessels,  which  similarly  pass  dorsad  along  the 
corresponding  visceral  arches,  and  which  unite  with  the 
dorsal  part  of  the  first  pair  to  form  the  primitive  aorta  of 
each  side.  Each  primitive  aorta  results,  therefore,  from  the 
confluence  of  all  the  visceral-arch  vessels  of  its  own  side 


Fig.  70. — Diagrams  illustrating  arrangement  of  primitive  heart  and  aortic 
arches  (modified  from  Allen  Thomson)  :  1,  vitelline  veins  returning  blood  from 
vascular  area ;  2,  venous  segment  of  heart-tube ;  3,  primitive  ventricle ;  4,  truncus 
arteriosus ;  5,  5,  upper  and  lower  primitive  aortee ;  5',  5',  continuation  of  double 
aortce  as  vessels  to  caudal  pole  of  embryo ;  6,  vitelline  arteries  returning  blood  to 
vascular  area. 

(Fig.  70).  The  two  aortse  afterward  become  merged  into  a 
single  trunk.  At  first  the  principal  branches  of  the  aorta 
arc  the  vitelline  arteries.  As  these  latter  vessels  become 
inconspicuous,  the  allantoic  or  umbilical  arteries  come  into 
prominence  as  the  chief  branches.  Indeed,  the  umbilical 
arteries  may  be  said  to  be  the  continuation  of  the  aorta,  since 
the  largest  part  of  the  blood-stream  is  diverted  into  them. 
The  aorta  proper  continues  in  the  median  line  as  the  caudal 
aorta,  which  latter  is  represented  in  the  adult  by  the  middle 
sacral  artery. 

So  far  the  arterial  system  of  the  fetus  presents  an  abso- 
lutely symmetrical  arrangement  (Fig.  70).  Changes  very 
soon  occur,  however,  which  lead  to  the  asymmetrical  condition 


THE  FETAL  ARTERIAL  SYSTEM. 


151 


found  in  the  adult.  These  changes  are  due  to  the  atrophy 
of  some  trunks  and  the  preponderance  of  others.  From  the 
point  where  the  dorsal  extremity  of  the  fourth  arch  joins 
the  fifth,  a  branch  passes  to  the  rudimentary  arm  (Fig.  71). 


Cointnon  carotid. 

Recurrent  laryngeal 
nerve. 


Right  subclavian. 


Innojiiinate  artery. 


Ascending  aorta. 


Vagus  nerve. 
External  carotid. 

Internal  carotid. 

Vertebral  artery. 

Arch  of  aorta. 
Left  subclavian. 

Ductus  arteriosus. 


Pulmonary  trunk. 


Descending  aorta. 


Fig.  71.— Diagram  illustrating  the  fate  of  the  aortic  arches  in  mammals  and  man 
(modified  from  Rathke). 

The  first  and  second  arches,  except  their  ventral  and  dorsal 
limbs,  undergo  atrophy.  The  ventral  limbs  of  the  first  and 
second  arches  persist  and  become  the  external  carotid  artery, 
while  their  dorsal  extremities,  with  the  third  visceral-arch 
vessel,  become  the  internal  carotid  artery.  The  ventral  stem 
of  the  third  arch  constitutes  the  common  carotid.  The  right 
fourth-arch  vessel  becomes  the  right  subclavian,  its  stream  of 
blood  being  conveyed  to '  the  arm  by  the  branch  which  has 
taken  its  origin  from  the  point  of  junction  of  the  dorsal  ends 
of  the  fourth  and  fifth  arches.     This  latter  branch  is  there- 


152  TEXT-BOOK  OF  EMBRYOLOGY. 

fore  the  continuation  of  the  subclavian.  The  ventral  seg- 
ment of  the  right  fourth  arch  would  be  represented  in  the 
adult  by  the  innominate  artery.  The  fourth  arch  of  the  left 
side  assumes  a  lower  position  ;  sinking  into  the  thorax,  it  be- 
comes the  arch  of  the  aorta.  Since  the  right  fifth  arch  becomes 
atrophic,  the  dorsal  end  of  the  right  fourth-arch  vessel — 
the  future  right  subclavian  artery — loses  its  connection  with 
the  primitive  aorta,  and  the  latter  now  appears  as  the  con- 
tinuation of  the  left  fourth  arch.  The  ventral  stem  of  the 
left  third  arch,  which  becomes  the  future  left  common  carotid, 
and  also  the  left  subclavian,  which  arises  from  the  posterior 
or  dorsal  end  of  the  left  fourth  arch,  are  now  branches  of 
the  arch  of  the  aorta.  When  the  truncus  arteriosus  becomes 
divided  into  the  aorta  and  the  pulmonary  artery,  the  left 
fifth-arch  vessel  is  the  only  one  of  the  branches  of  the  truncus 
that  falls  to  the  pulmonary  artery,  all  the  other  visceral-arch 
vessels  being  connected  with  the  aorta.  The  left  fifth  vis- 
ceral-arch vessel  therefore  is  represented  in  the  adult  by  the 
pulmonary  artery.  The  fetal  lungs  being  impervious,  only  a 
very  small  part  of  the  blood  of  the  pulmonary  artery  is  sent 
to  them.  The  larger  portion  of  the  blood  passes  from  the 
pulmonary  artery  to  the  aorta  through  a  communicating 
trunk,  the  ductus  arteriosus,  which  becomes  impervious  after 
birth  with  the  establishment  of  the  proper  pulmonary  circu- 
lation. 

These  transformations  afford  an  explanation  of  the  different 
relations  of  the  recurrent  laryngeal  nerves  of  the  two  sides. 
At  first  they  are  symmetrically  arranged.  The  pneumo- 
gastric  nerve,  as  it  crosses  the  fourth  visceral-arch  vessel, 
gives  off  the  recurrent  laryngeal  nerve,  the  latter  winding 
around  the  artery  from  before  backward  on  its  way  to  the 
larynx.  When  the  left  fourth  arch  becomes  the  arch  of  the 
aorta  and  sinks  into  the  chest,  the  nerve  is  carried  with  it ; 
hence  after  this  time,  the  left  nerve  is  found  winding  around 
the  arch  of  the  aorta. 

Anomalous  arrangements  of  the  branches  of  the  aortic 
arch,  as  well  as  of  the  arch  itself,  are  referable  to  anomalous 
development  of  the  original  system  of  visceral-arch  vessels. 


THE  FETAL    VENOUS  SYSTEM.  153 

For  example,  if  the  right  fourth  arch,  which  usually  becomes 
the  right  subclavian  artery,  be  suppressed  from  its  origin  to 
the  point  where  the  artery  for  the  right  upper  extremity  is 
given  off,  the  blood  must  find  its  way  into  the  latter  vessel 
through  the  dorsal  stem  of  the  fourth  arch,  and  this  dorsal 
stem  will  then  become  the  right  subclavian  artery.  In  such 
case,  the  right  subclavian  of  the  adult  will  be  found  to  arise 
from  the  left  extremity  of  the  arch  of  the  aorta  and  to  pass 
obliquely  upward  to  the  right  side  of  the  neck  behind  the 
trachea  and  the  esophagus. 

THE  FETAL  VENOUS  SYSTEM. 

The  venous  system  of  the  embryo  presents  several  suc- 
cessive phases,  corresponding  in  part  with  the  various  stages 
in  the  evolution  of  the  arterial  system.  The  first  trunks  to 
appear  are  the  vitelline  veins.  These  vessels  have  their  origin 
in  the  vascular  area  on  the  wall  of  the  yolk-sac  in  the  manner 
already  described  in  connection  with  the  vitelline  circulation. 
The  two  vitelline  or  omphalomesenteric  veins,  which  result 
from  the  convergence  of  all  the  venous  trunks  of  the  vas- 
cular area,  follow  the  vitelline  duct  into  the  body  of  the 
embryo  through  the  still  widely  open  umbilical  aperture  and 
take  their  course  headward  along  the  intestinal  canal  to  open 
into  the  caudal  end  of  the  primitive  heart-tube  (Fig.  70, 1, 1). 
At  a  later  period  they  open  into  the  sinus  venosus  of  the  heart, 
and  still  later,  when  the  sinus  venosus  becomes  a  part  of  the 
general  atrial  cavity,  into  the  atrium  itself.  Near  their  termi- 
nation these  veins  communicate  with  each  other  by  anastomos- 
ing trunks  that  encircle  the  future  duodenal  region  of  the  in- 
testinal tube.  As  the  yolk-sac  diminishes  in  size  and  impor- 
tance, the  vitelline  veins  decrease  in  caliber,  and  the  umbilical 
veins,  conveying  blood  from  the  allantois  and  subsequently 
from  the  placenta,  functionally  replace  them.  The  proximal 
parts  of  the  vitelline  veins  have  an  important  connection 
with  the  circulation  of  the  liver,  as  will  be  seen  hereafter. 

The  umbilical  veins,  which  are  developed  in  the  mesodermic 
tissue  of  the  allantois,  pass  from  the  placenta  along  the 
umbilical  cord  and,  entering  the  fetal  body  at  the  umbilicus. 


154  TEXT-BOOK  OF  EMBRYOLOGY. 

run  at  first  along  the  lateral,  and  later  along  the  ventral, 
wall  of  the  abdomen  toward  the  heart.  Meanwhile  there 
have  been  established  a  pair  of  venous  trunks,  the  primitive 
jugular  veins  (Fig.  72,  A),  to  return  the  blood  from  the  head 
and  the  upper  part  of  the  trunk ;  and  a  second  pair,  the  car- 
dinal veins,  which  bring  the  blood  from  the  lower  part  of 
the  trunk,  and  especially  from  the  primitive  kidneys.  The 
primitive  jugular  vein — which  represents  the  external  jug- 
ular of  the  adult — passing  downward  along  the  dorsal  re- 
gion of  the  neck,  meets  the  cardinal  vein  of  its  own  side  and 
imites  with  it  near  the  heart,  the  short  thick  trunk  thus 
formed  being  the  duct  of  Cuvier.  The  right  and  left  ducts 
of  Cuvier  converge  and  open  together  into  the  sinus  venosus 
(sinus  reuniens)  of  the  heart,  which  also  now  receives  the  vitel- 
line veins  and  the  umbilical  veins.  Upon  the  development  of 
the  upper  and  the  lower  limbs,  the  cardinal  vein  appears  as 
if  formed  by  the  confluence  of  the  internal  and  external  iliac 
veins,  while  the  primitive  jugular  below  the  entrance  of  the 
subclavian  vein  is  designated,  with  the  duct  of  Cuvier,  the 
superior  vena  cava,  since,  owing  to  the  preponderance  of  the 
jugular  over  the  cardinal  vein,  the  Cuvierian  duct  appears 
to  be  a  direct  continuation  of  the  jugular.  At  this  time, 
then,  there  are  two  superior  vense  cavse,  the  terminal  parts 
of  which,  however,  are  not  exactly  symmetrical,  since  the 
left  passes  around  the  dorsal  or  posterior  wall  of  the  atrium, 
owing  to  the  rotation  of  the  heart  from  riglit  to  left. 

The  lower  venous  trunks  likewise  ]>resent  a  symmetrical 
arrangement.  The  bilateral  symmetry  of  this  stage  of  the 
venous  system,  while  permanent  in  fishes,  becomes  modified 
in  man  to  produce  the  familiar  asymmetrical  condition  of  the 
adult  venous  trunks  by  two  factors  principally — first,  the 
development  of  an  unpaired  vessel  which  is  to  constitute  a 
part  of  the  inferior  vena  cava,  and  second,  the  atrophy  of 
certain  vessels  and  parts  of  vessels  with  a  consequent  diver- 
sion of  the  major  part  of  their  blood-stream  into  other  chan- 
nels. Associated  with  these  alterations  is  the  evolution  of  a 
special  set  of  blood-vessels,  the  portal  venous  system,  for  the 
supply  of  the  developing  liver.      The  development  of  the 


THE  FETAL    VENOUS  SYSTEM. 


155 


portal  system,  however,  may  be  deferred  for  separate  con- 
sideration (see  page  161). 

When  the  sinus  venosus  becomes  a  part  of  the  atrium — 
constituting  that  part  of  the  wall  of  the  adult  auricle  which 
is  destitute  of  musculi  pectinati — the  two  ducts  of  Cuvier,  or 
the  superior  cavse,  as  well  as  the  veins  from  the  abdominal 
viscera,  open  by  separate  orifices  into  the  atrial  cavity.  An 
unpaired  vessel  now  develops  below  the  heart  in  the  tissue  be- 


FiG.  72.— Schematic  representation  of  the  human  venous  system,  with  three 
successive  stages  of  development  (after  Hertwig) :  1,  vena  cava  inferior;  2,  cardi- 
nal veins;  3,  vena  azygos  major;  4,  vena  azygos  m.inor;  5,  renal  veins;  6,  external 
iliac  vein;  7,  internal  iliac  vein;  8  and  9,  common  iliac  veins;  10,  early  superior 
venEecavse;  11,  ducts  of  Cuvier;  12,  primitive  jugular  vein;  13,  internal  jugular ; 
14,  suhclavian  vein  ;  15  and  16,  right  and  left  innominate  veins ;  17,  vena  cava  su- 
perior ;  18,  coronary  vein ;  19,  duct  of  Arantius  ;  20,  hepatic  veins. 

tween  the  primitive  kidneys  (Fig.  72,  A,  1).  This  constitutes 
the  upper  or  cardiac  segment  of  the  inferior  vena  cava.  The 
lower  extremity  of  this  trunk  anastomoses  by  two  transverse 
branches  with  the  right  and  the  left  cardinal  veins  (Fig.  72, 
B).  The  cardinal  veins  of  the  two  sides  are  further  con- 
nected by  a  transverse  trunk  at  their  lower  extremities  and 
by  one  that  passes  across  the  vertebral  column  just  below  the 
heart.     In  like  manner  the  two  superior  venae  cavse  commu- 


]56  TEXT-BOOK  OF  EMBRYOLOGY. 

nicate  with  each  other  by  a  transverse  vessel,  the  transverse 
jugular  vein,  at  the  upper  part  of  the  thorax,  above  the  arch 
of  the  aorta.  With  the  exception  of  the  unpaired  trunk 
which  is  destined  to  constitute  the  upper  part  of  the  inferior 
vena  cava,  the  arrangement  of  the  veins  at  this  time  is  abso- 
lutely symmetrical.  The  apparently  meaningless  asymmetry 
of  the  adult  venous  trunks  is  easily  accounted  for  if  one 
notes  the  alterations  in  the  course  of  the  blood-current  which 
now  occur. 

The  blood-stream  of  the  left  superior  vena  cava  gradually 
becomes  entirely  diverted  into  the  right  cava  through  the 
transverse  jugular  vein,  and  the  part  of  the  left  cava  below 
this  point,  being  now  functionless,  shrivels  to  an  impervious 
cord  (Fig.  72,  C).  This  cord  or  strand  of  tissue,  the  rem- 
nant of  the  left  superior  cava,  is  found  in  postnatal  life,  in 
front  of  the  root  of  the  left  lung,  embedded  in  a  fold  of  the 
serous  layer  of  the  pericardium,  the  so-called  vestigial  fold 
of  Marshall.  Since  the  left  superior  vena  cava  receives,  near 
its  termination  in  the  auricle,  the  large  coronary  vein,  which 
returns  the  greater  part  of  the  blood  from  the  heart-wall, 
this  proximal  extremity  of  the  left  cava  persists  as  the 
coronary  sinus  of  the  heart.  The  transverse  communicating 
trunk — the  transverse  jugular  vein — and  the  part  of  the  left 
cava  above  it  now  constitute  the  left  innominate  vein,  the 
course  of  which  from  left  to  right  is  thus  explained.  The 
left  superior  vena  cava  of  the  fetus  is  represented  in  the  adult, 
therefore,  by  the  sinus  coronarius,  by  the  atrophic  impervious 
cord  lying  in  Marshall's  vestigial  fold,  by  the  vertical  part 
of  the  left  innominate  vein  and  by  a  part  of  the  left  superior 
intercostal  vein. 

The  lowest  connecting  branch  between  the  cardinal  veins 
enlarges  and  conveys  to  the  right  cardinal  vein  the  blood 
from  the  left  internal  and  external  iliac  veins  (Fig.  72),  in 
consequence  of  which  the  part  of  the  left  cardinal  vein 
below  the  kidney  undergoes  atrophy  and,  finally,  complete 
obliteration.  The  newly-formed  transverse  trunk  is  the  left 
common  iliac  vein.  The  part  of  each  cardinal  vein  above 
the  renal  region  suffers  an  arrest  in  growth,  in  consequence 


THE  FETAL    VENOUS  SYSTEM.  157 

of  which  the  blood  is  diverted  from  these  veins  into  the 
transverse  anastomosing  branches  before  mentioned  as  con- 
necting the  respective  cardinal  veins  with  the  lower  end 
of  the  unpaired  caval  trunk  (Fig.  72,  B  and  C,  5).  As 
a  result,  the  lower  half  of  the  right  cardinal  vein,  now 
receiving  at  its  distal  end  the  two  common  iliac  veins,  be- 
comes directly  continuous  with  the  unpaired  caval  trunk,  and 
with  it  constitutes  the  inferior  vena  cava.  The  inferior  vena 
cava,  therefore,  is  partly  an  independently  formed  structure 
and  is  partly  the  greatly  developed  lower  half  of  the  right 
cardinal  vein.  The  upper  half  of  the  right  cardinal  vein, 
conveying  now  a  relatively  small  part  of  the  blood-stream, 
becomes  the  vena  azygos  major,  the  termination  of  which  in 
the  superior  vena  cava  is  explicable  when  it  is  borne  in  mind 
that  the  cardinal  and  the  primitive  jugular  veins,  by  their 
confluence,  form  the  duct  of  Cuvier. 

While  no  part  of  the  right  cardinal  vein  suffers  complete 
effacement,  the  left  one,  in  a  part  of  its  course,  entirely  dis- 
appears. All  the  blood  of  the  left  external  and  internal  iliac 
veins  being  transported  to  the  right  side  of  the  body  through 
the  lowest  transverse  trunk — that  is,  the  newly-formed  left 
common  iliac  vein — the  part  of  the  left  cardinal  vein  below 
the  kidney  retrogrades  and  disappears.  The  part  of  the  left 
cardinal  above  the  renal  region  lagging  behind  in  growth, 
the  blood  from  the  left  kidney  is  conveyed  to  the  inferior 
vena  cava  by  the  transverse  trunk  that  connects  the  cardinal 
veins  in  the  renal  region  ;  this  transverse  trunk  becomes,  there- 
fore, the  left  renal  vein.  Since  the  spermatic  veins  originally 
emptied  into  the  cardinal  veins,  it  is  found,  after  these  trans- 
formations, that  the  right  spermatic  opens  into  the  inferior 
vena  cava,  while  the  left  spermatic  is  a  tributary  of  the  left 
renal  vein.  Some  anatomists,  indeed,  regard  the  left  sper- 
matic vein  as  the  representative  of  the  lower  part  of  the  left 
cardinal  vein  of  the  fetus. 

As  the  left  renal  vein  develops  into  the  channel  for  the 
major  part  of  the  blood  from  the  left  kidney,  the  portion  of 
the  left  cardinal  vein  above  this  point  remains  an  incon- 
spicuous vessel,  and  that  part  of  it  intervening  between  the 


158 


TEXT-BOOK  OF  EMBRYOLOGY. 


duct  of  Cuvier  and  the  cross  branch  (Fig.  72,  C,  4)  situated 
immediately  below  the  heart  undergoes  total  obliteration.  The 
blood  ascending  through  the  persisting  part  of  the  left  cardi- 
nal vein  must  therefore  pass  across  to  the  upper  part  of  the 
right  cardinal  vein,  now  the  vena  azygos  major;  and  the 
pervious  portion  of  the  left  cardinal  vein,  with  the  trans- 
verse trunk  referred  to,  constitutes  the  vena  azygos  minor. 

THE  FORMATION  OF  THE  PERICARDIUM,  THE  PLEUR/E, 
AND  THE  DIAPHRAGM. 

The  development  of  the  pericardium  is  so  intimately  re- 
lated with  that  of  the  pleurae  and  of  the  diaphragm  that  an 
account  of  it  involves  a  description  of  the  evolution  of  those 
structures.  By  way  of  facilitating  a  comprehension  of  the 
rather  complicated  details  of  the  process,  the  reader  is  re- 
minded that  the  tube  which  constitutes  the  primitive  heart 
is  formed  by  the  coalescence  of  the  two  tubes  produced 
within  the  splanchnic  mesoderm,  and  that  this  tube  and  also, 
for  a  time,  the  heart  resulting  from  it,  are  embedded  within 
the  ventral  mesentery ;  and,  further,  that  the  part  of  the 
ventral  mesentery  connecting   the   heart   with   the   ventral 


Fig.  73.— Diagrammatic  cross-sections  of  the  body  of  the  embryo  in  the  region 
of  the  heart  at  level  of  future  diaphragm  :  a,  esophageal  segment  of  gut-tract ;  6, 
dorsal  mesentery;  c,  mesocardiunvposterius;  d,  mesocar^um  anterius;  e,  begin- 
ning of  septum  transversum,  containing  vitelline  and  allantoic  veins;  /,  septum 
transversum  ;  g,  thoracic  prolongation  of  abdominal  cavity  ;  nc,  neural  canal. 

body-wall  is  the  mesocardium  anterius,  while  the  fold  passing 
from  the  heart  to  the  gut-tract  is  the  mesocardium  posterius 
(Fig.  73,  A,  and  Fig.  61,  C).  The  space  between  the  heart 
and  the  body-wall  is  a  part  of  the  body-cavity  or  coelom 
(throat-cavity  of  Kolliker,  parietal    cavity  of  His).     The 


THE  FORMATION  OF  THE  PERICARDIUM.  159 

first  indication  of  the  separation  of  this  space  from  the  future 
abdominal  cavity  is  furnished  by  the  appearance  of  a  trans- 
verse ridge  of  tissue  growing  from  the  ventral  and  lateral 
aspects  of  the  body-wall.  This  mass  is  the  septum  trans- 
versum.  It  bears  an  important  relation  to  the  course  of  the 
vitelline  and  the  umbilical  veins.  As  the  veins  diverge 
from  the  body-wall  to  reach  the  heart,  they  carry  with  them, 
as  it  were,  the  parietal  layer  of  the  mesoderm  in  which  they 
are  embedded,  forming  on  each  side  a  fold  that  projects  me- 
sialiy  and  dorsally  (Fig.  73,  B  and  C),  the  two  folds  ap- 
proaching and  finally  meeting  with  the  ventral  mesentery 
in  the  median  plane.  The  septum  transversum  thus  formed 
contains  in  the  region  nearer  the  intestine  a  mass  of  em- 
bryonal connective  tissue  which  is  called  the  liver-ridge  or 
prehepaticus  from  the  fact  that  the  developing  liver  grows 
into  it.  Since  the  septum  transversum,  exclusive  of  the 
so-called  liver-ridge,  is  the  primitive  diaphragm,  it  will  be 
seen  that  the  liver,  in  the  early  stages  of  its  growth,  is  inti- 
mately associated  with  the  anlage  ^  of  the  diaphragm.  The 
septum  transversum  partially  divides  the  body-cavity  into  a 
pericardiothoracic  and  an  abdominal  part,  as  shown  in  Fig. 
73,  B  and  C.  Near  the  dorsal  wall  of  the  trunk,  on  each 
side  of  the  intestine  and  its  mesentery,  the  septum  is  want- 
ing, and  thus  the  two  spaces  communicate  wath  each  other 
by  openings  that  are  known  as  the  thoracic  prolongations  of 
the  abdominal  cavity.  At  this  stage,  then,  the  four  great 
serous  sacs  of  the  body,  the  two  pleural,  the  pericardial,  and 
the  abdominal,  are  indicated,  but  are  still  in  free  communi- 
cation with  each  other. 

The  pericardial  cavity  is  the  first  one  of  these  to  be  closed 
off;  subsequently  the  pleural  sacs  are  delimited  from  the 
abdominal  space.  Just  as  the  transverse  septum,  which 
partly  forms  the  floor  of  the  thoracic  cavity,  holds  an  im- 
portant relation  to  the  course  of  the  vitelline  and  the  umbili- 
cal veins  on  their  way  to  the  heart,  so  is  a  vertical  septum 

^  Anlage,  a  German  word  signifying  groundwork,  or,  in  embryology,  the 
first  crude  outline  of  an  organ  or  part,  has  come  into  use  in  English  writ- 
ings upon  the  subject  because  there  is  no  exact  English  equivalent  for  it. 


160 


TEXT-BOOK  OF  EMBRYOLOGY. 


(Fig.  74,  A,  b)  which  separates  the  pericardial  space  from  the 
pleural  spaces  associated  with  the  position  of  a  large  vein. 
This  vein,  the  duct  of  Cuvier,  formed  in  the  upper  part  of 
the  thorax  by  the  confluence  of  the  cardinal  and  the  ji\gular 
veins,  lies  at  first  near  the  dorsal  body- wall  and  then  along 
its  lateral  aspect.  In  the  latter  position  it  encroaches  upon 
the  pleuropericardial  space  and  is  covered  by  the  somatic  or 
parietal  mesoderm  (Fig.  74,  A).     It  is  this  inwardly  project- 


A  B  C 

Fig.  74.— Diagrammatic  cross-sections  of  the  body  of  the  embryo  in  the  region 
of  the  heart  entirely  above  the  level  of  the  diaphragm :  a,  esophagus ;  6,  pleuro- 
pericardial fold  containing  duct  of  Cuvier  ;  c,  pleuropericardial  space  ;  d,  meso- 
cardium  posterius  ;  e,  mesocardium  anterius  ; /,  lung;  gr,  pleural  cavity;  7i,  peri- 
cardial cavity. 

ing  vertical  fold  of  serous  membrane  containing  the  duct  of 
Cuvier  which  constitutes  the  pleuropericardial  fold  and  the 
appearance  of  which  initiates  the  division  of  the  thoracic  cav- 
ity into  two  spaces,  one  for  the  heart  and  one  for  the  lungs. 
The  pleuropericardial  fold  continues  to  grow  toward  the  me- 
dian plane  of  the  body  until  it  meets  the  mesocardium  pos- 
terius (Fig.  74,  _B),  with  w^hich  it  fuses,  thus  completing  the 
pericardial  sac  (A)  and  isolating  it  from  the  pleural  space  {g). 
The  heart  is  still  relatively  very  large  and  occupies  the 
greater  part  of  the  thoracic  cavity,  leaving  only  a  compara- 
tively small  space,  situated  dorsally,  for  the  accommodation 
of  the  developing  lungs.  This  latter  space,  as  previously 
mentioned,  remains  for  a  long  time  in  communication  with 
the  abdominal  cavity  by  the  two  thoracic  prolongations  of 
the  latter,  which  lie  one  on  each  side  of  the  intestinal  tube 
and  its  mesentery  (Fig.  73,  C,  and  Fig.  74,  A,  B).  Kefer- 
ence  to  Fig.  74,  B,  will  show  that  the.se  tube-like  spaces  are 
enclosed  completely  by  serous  membrane  and  that  they  are 


THE  FORMATION  OF  THE  PERICARDIUM.  161 

entirely  distinct  from  each  other.  It  is  evident  also,  that  the 
mesial  wall  of  each  space  is  constituted  by  the  raesocardium 
posterius  and  the  dorsal  mesentery.  The  lungs  first  appear 
as  two  little  sacs,  connected  by  a  common  pedicle,  the  future 
trachea,  with  the  upper  end  of  the  esophagus.  As  they  grow 
downward  in  front  of  the  esophagus  and  in  contact  with  it, 
they  push  the  serous  membrane  before  them  carrying  it  away 
from  the  esophagus  (Fig.  74,  B),  and  thus  they  acquire  an 
investment  of  serous  membrane,  which  is  the  visceral  layer 
of  the  pleura.  The  layer  of  serous  membrane  in  contact 
with  the  body-wall  is  the  parietal  layer  of  the  pleura.  The 
lower  extremities  of  the  lungs  at  length  come  into  relation 
with  the  upper  surface  of  the  liver,  from  which  organ  they 
are  finally  separated  by  the  growth  of  two  folds,  the  pillars 
of  Uskow,  from  the  dorsolateral  region  of  the  body-wall. 
These  folds  or  ridges  project  forward  and  unite  with  the 
earlier  formed  septum  trans versum  to  complete  the  dia- 
phragm. So  far,  however,  the  diaphragm  is  merely  connective 
tissue,  the  muscular  condition  being  acquired  later  by  the 
ingrowth  of  muscular  substance  from  the  trunk.  Occasion- 
ally the  dorsal  or  younger  part  of  the  diaphragm  fails  to 
unite  with  the  ventral  or  older  fundament  on  one  side  of  the 
body,  leaving  an  aperture  through  which  a  portion  of  the 
intestine  may  pass  into  the  thoracic  cavity.  Such  a  condition 
constitutes  a  congenital  diaphragmatic  hernia. 

The  heart  and  its  pericardial  sac  occupy  the  greater  part 
of  the  thoracic  cavity,  while  the  lungs  are  merely  narrow 
elongated  organs  lying  in  the  dorsal  part  of  this  space  as 
shown  in  Fig.  74,  B.  As  the  lungs  increase  in  diameter, 
they  spread  out  ventrally  and  gradually  displace  the  parietal 
layer  of  the  pericardium  (Fig.  74,  B)  from  the  lateral  wall 
of  the  chest,  crowding  the  pericardium  forward  and  toward 
the  median  plane  of  the  body  (see  Fig.  74,  C)  until  finally 
the  adult  relationship  of  these  structures  is  established. 

THE  PORTAL  CIRCULATION. 

The  circulation  of  the  adult  liver  is  peculiar  in  that  the 
organ  is  supplied  not  only  with  arterial  blood  for  its  nutrition 
11 


162 


TEXT-BOOK  OF  EMBRYOLOGY. 


but  receivGvS  also  venous  blood  laden  with  certain  products  of 
digestion  obtained  from  the  alimentary  tract,  the  spleen,  and 
the  pancreas.  This  venous  blood  enters  the  liver  through 
the  portal  vein  and  is  designed  to  supply  to  the  gland  the 
materials  for  the  performance  of  its  special  functions. 


Fig.  75.— Four  successive  stages  in  the  development  of  the  portal  venous  sys- 
tem (from  Tourueux,  after  His) :  1,  outline  of  liver  ;  2,  duodenum  ;  3,  ainus  veno- 
sus;  4,  4,  umbilical  veins ;  5,  5,  A,  vitelline  veins,  which  in  B  and  Care  connected 
by  the  annular  sinus;  6,  superior  vena  cava;  6',  coronary  vein;  7,  portal  vein;  8, 
ductus  venosus ;  9,  9,  vense  hepaticse  revehentes ;  10, 10,  venae  hepaticte  advehentes. 

As  might  be  expected  from  the  fact  that  the  liver  is  an 
appendage  of,  and  a  direct  outgrowth  from,  the  intestinal 
canal,  it  receives  its  blood-supply,  in  the  early  stages  of  its 
development,  from  the  vessels  that  supply  the  primitive 
intestine,  that  is,  from  the  vitelline  veins.  These  veins,  on 
their  way  to  the  heart,  pass  along  the  intestinal  canal  and 
are  connected  with  each  other  in  the  region  of  the  future 


THE  PORTAL   CIRCULATION.  163 

duodenum  by  trunks  that  encircle  the  bowel,  these  connect- 
ing vessels  collectively  constituting  the  annular  sinus  (Fig. 
75,  B  and  C).  The  liver  originates  from  a  small  diver- 
ticulum which  is  evaginated  from  the  ventral  wall  of  the  in- 
testinal canal.  Growing  forward  between  the  folds  of  the 
ventral  mesentery,  this  little  tubular  sac  divides  and  sub- 
divides so  as  to  produce  a  gland  of  the  compound  tubular 
type.  The  developing  liver  is  from  the  first  in  close  relation 
with  the  vitelline  veins  and  their  ring-like  anastomosing 
branches,  and  receives  its  blood-supply  from  the  latter  through 
vessels  that  are  known  as  the  venae  hepaticae  advehentes 
(Fig.  75,  10,  10).  These  afferent  vessels  break  up  within 
the  liver  into  a  system  of  capillaries,  from  which  the  blood 
passes  through  the  efferent  vessels,  the  venae  hepaticae  reve- 
hentes,  into  the  terminal  parts  of  the  vitelline  veins.  Thus 
a  part  of  the  blood  of  the  vitelline  veins  is  diverted  to  the 
liver  and,  after  circulating  through  that  organ,  is  returned  to 
them  further  on  to  be  conveyed  to  the  heart.  As  the  liver, 
with  its  increasing  development,  requires  more  and  more 
blood,  the  entire  blood-stream  of  the  vitelline  veins  passes  to 
it,  and  the  parts  of  these  veins  between  the  vense  hepaticse 
advehentes  and  the  vense  hepaticfe  revehentes  become  obliter- 
ated (Fig.  75,  B  and  C).  The  vitelline  veins,  therefore, 
leave  the  intestinal  canal  at  the  duodenal  region  and  traverse 
the  liver  on  their  way  to  the  heart.  In  this  early  stage  of 
the  development  of  the  liver,  then,  it  receives  its  nutrition  from 
the  yolk-sac,  through  the  vitelline  veins. 

When  the  yolk-sack  undergoes  retrogression,  as  it  does 
about  the  fifth  week,  the  liver  must  draw  upon  the  allantoic 
and  the  placental  vessels  for  its  nutrition.  To  do  this  it 
must  acquire  connection  with  the  umbilical  veins.  The  latter 
vessels  pass  upward  from  the  umbilicus  along  the  ventral 
wall  of  the  body  and  empty  into  the  sinus  venosus  of  the 
heart  above  the  site  of  the  liver  (Fig.  75,  A,  4,  4).  The  two 
umbilical  veins  fuse  to  form  one,  and  this  one  effects  commu- 
nications beneath  the  liver  with  the  vense  hepaticse  advehentes 
from  the  vitelline  veins.  As  the  needs  of  the  liver  exceed  the 
capacity  of  the  vitelline  veins,  more  and  more  of  the  blood  of 


164  TEXT-BOOK  OF  EMBRYOLOGY. 

the  umbilical  vein  is  sent  to  it,  until  finally  all  the  blood  of  the 
latter  vein  passes  into  the  liver  and  reaches  the  heart  through 
the  terminal  part  of  the  left  vitelline  vein.  (The  left  vitel- 
line vein  very  early  begins  to  predominate  over  the  right.) 
The  part  of  the  umbilical  vein  above  the  liver  undergoes 
atrophy  and  disappears.  Although,  meanwhile,  the  yolk-sac 
has  dwindled,  the  vitelline  veins  persist,  in  part,  since  they 
receive  blood  from  the  walls  of  the  alimentary  tract.  The 
liver  now,  in  this  second  stage  of  its  development,  receives  blood 
from  two  sources,  the  abdominal  viscera  and  the  placenta. 

As  previously  indicated,  the  proximal  half  of  the  inferior 
vena  cava  develops  as  an  unpaired  vessel  connected  with  the 
primitive  heart.  It  opens  above  into  the  left  vitelline  vein. 
In  a  short  time  it  far  outstrips  the  latter  in  growth  and,  with 
its  extension  downward,  the  point  of  union  of  the  two  ves- 
sels is  carried  downward  toward  the  liver,  the  vitelline  vein 
becoming  larger  and  constituting  now  the  hepatic  vein. 
Meanwhile  the  volume  of  blood  flowing  through  the  umbili- 
cal vein  has  increased  to  such  an  extent  that  the  liver  is  no 
longer  able  to  transmit  it  to  the  inferior  vena  cava,  and  con- 
sequently a  communication  is  established  between  these  two 
vessels  on  the  under  surface  of  the  liver.  The  connecting 
branch  is  the  ductus  venosus  or  ductus  Arantii,  The  blood 
of  the  umbilical  vein  is  divided,  therefore,  into  two  streams — 
one  that  enters  the  inferior  vena  cava  directly  through  the 
ductus  venosus  and  one  that  traverses  the  liver  on  its  way  to 
the  cava. 

The  portal  vein  results  from  the  persistence  of  a  part  of 
the  vitelline  veins.  The  vitelline  veins,  as  we  have  seen, 
anastomose  with  each  other  by  two  ring-like  branches  that 
encircle  the  duodenum.  The  right  half  of  the  lower  ring 
and  the  left  half  of  the  upper  one  atrophy,  so  that  the  blood 
of  the  vitelline  veins  makes  its  way  to  the  liver  through  the 
left  half  of  the  lower  ring  and  the  right  half  of  the  upper 
one  (Fig.  75,  D).  This  single  vessel  constitutes  the  portal 
vein,  and  its  course,  therefore,  is  backward  around  the  left 
side  of  the  duodenum  and  then  to  the  right.  So  long  as  the 
yolk-sac  is  present,  the  vein  receives  blood  both  from  it  and 


FINAL  'STAGE  OF  THE  FETAL   VASCULAR  SYSTEM.   165 

from  the  walls  of  the  intestine.  After  the  disappearance  of 
the  yolk-saCj  the  intestinal  and  the  visceral  veins  are  the  sole 
tributaries  of  the  portal  vein. 

THE  FINAL  STAGE  OF  THE  FETAL  VASCULAR  SYSTEM. 

The  circulation  of  the  fetus  at  birth  and  the  changes  ensu- 
ing immediately  thereafter  may  now  be  easily  understood. 
The  fetal  blood  being  sent  to  the  placenta  through  the  hypo- 
gastric or  umbilical  arteries,  receives  oxygen  there  and  is 
returned  to  the  body  of  the  fetus  through  the  umbilical  vein. 
The  latter  vessel  takes  its  course  upward  along  the  ventral 
wall  of  the  abdomen  to  the  under  surface  of  the  liver,  lying 
here  in  the  anterior  part  of  the  longitudinal  fissure.  In  this 
position  the  blood-stream  of  the  umbilical  vein  is  divided 
into  two  parts,  one  of  which  unites  with  the  fetal  portal 
vein  to  enter  the  liver,  while  the  other  passes  through  the 
ductus  venosus  directly  to  the  inferior  vena  cava.  The 
blood  which  enters  the  liver,  after  traversing  that  organ, 
reaches  the  inferior  vena  cava  through  the  hepatic  veins. 
Thus,  in  the  one  case  directly,  in  the  other  case  by  passing 
through  the  liver,  all  the  placental  blood  reaches  the  inferior 
vena  cava  and  passes  on  to  the  right  auricle  of  the  heart. 

From  the  right  auricle  the  blood  passes  through  the  for- 
amen ovale  to  the  left  auricle,  and  thence,  through  the  mitral 
orifice,  to  the  left  ventricle.  Being  driven  from  the  left  ven- 
tricle into  the  aorta,  it  is  conveyed  through  the  branches  of 
the  aortic  arch  to  the  head  and  the  upper  extremities.  Find- 
ing its  way  into  the  veins  of  these  parts,  it  is  returned,, 
through  the  superior  vena  cava,  to  the  right  auricle,  from 
which  cavity  it  passes,  through  the  tricuspid  orifice,  into 
the  right  ventricle.  From  the  right  ventricle  it  goes  into 
the  pulmonary  artery.  Since  the  lungs  are  not  as  yet  per- 
vious, or  but  very  slightly  so,  the  current  is  deflected  almost 
entirely  through  the  ductus  arteriosus  to  the  descending  aorta 
instead  of  going  to  the  lungs.  Some  of  the"  blood  of  the 
descending  aorta  is  distributed  to  the  various  parts  of  the 
body  below  the  position  of  the  heart,  while  some  of  it  is 
sent  through  the  hypogastric  or  umbilical  arteries  to  the  pla- 


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uricle. 
ovale, 
which 
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FINAL  STAGE  OF  THE  FETAL    VASCULAR  SYSTEM.  167 

centa  for  aeration.  It  is  evident  that  no  part  of  the  fetal 
blood,  except  that  in  the  umbilical  vein,  is  entirely  pure,  the 
venous  and  the  arterial  blood  being  always  more  or  less 
mixed. 

With  the  detachment  of  the  placenta  at  birth,  several 
marked  alterations  occur.  The  circulation  through  the 
umbilical  vein  ceases,  that  part  of  this  vessel  which  inter- 
venes between  the  umbilicus  and  the  portal  fissure  of  the 
liver  becoming,  in  consequence,  an  impervious  fibrous  cord, 
the  round  ligament  of  the  liver.  The  ductus  venosus  like- 
wise suffers  obliteration.  Since  the  lungs  now  assume  their 
proper  function  of  respiration,  the  communication  between 
the  right  and  the  left  sides  of  the  heart  and  also  that  between 
the  pulmonary  artery  and  the  aorta  cease.  Hence,  the  re- 
spective avenues  for  these  communications,  the  foramen  ovale 
and  the  ductus  arteriosus,  become  obsolete.  There  being  no 
further  need  for  the  hypogastric  (umbilical)  arteries,  the  cir- 
culation through  them  ceases,  and  they  become  mere  cords 
of  fibrous  tissue,  whose  presence  is  evidenced  by  two  ridges 
in  the  peritoneum  on  the  inner  surface  of  the  anterior  wall 
of  the  abdomen.  The  proximal  parts  of  these  arteries  persist, 
however,  as  the  superior  vesical  arteries. 


CHAPTER   XI. 
THE    DEVELOPMENT  OF  THE   DIGESTIVE  SYSTEM. 

The  adult  digestive  system  consists  of  the  moutli  with  its 
accessory  organs,  the  teeth,  the  tongue,  and  the  salivary  glands ; 
of  the  pharynx,  the  esophagus,  the  stomach,  and  the  small  and 
the  large  intestine,  inckiding  also  the  important  glandular 
organs,  the  liver  and  the  pancreas.  Notwithstanding  the 
apparent  complexity  of  its  structure,  the  alimentary  tract 
may  be  regarded  as  a  tube,  certain  regions  of  which  have 
become  specialized  in  order  to  adapt  them  to  the  perform- 
ance of  their  respective  functions,  the  salivary  glands,  the 
liver,  and  the  pancreas  being  highly  differentiated  evagina- 
tions  of  its  walls.  While  in  man  and  in  the  higher  verte- 
brates the  tube  is  thrown  into  coils  by  reason  of  its  excessive 
length,  in  the  lower-type  animals  it  is  much  more  simple  in 
its  arrangement.  For  example,  in  certain  fishes  and  in  some 
amphibians  the  alimentary  tract  has  the  form  of  a  slightly 
flexuous  tube,  the  deviations  from  the  simple  straight  canal 
being  few  and  insignificant,  and  the  stomach  being  repre- 
sented by  a  local  dilatation  of  the  tube. 

The  simple  condition  obtaining  in  the  representatives  of 
the  animal  kingdom  referred  to  above  suggests  the  likewise 
simple  fundamental  plan  of  the  human  embryonic  gut-tract. 
There  is,  in  fact,  a  period  in  development  when  the  gut-tract 
of  the  human  embryo  has  the  form  of  a  simple  straight  tube. 
The  processes  incident  to  the  formation  of  this  tube  mark 
the  earliest  stages  of  the  development  of  the  alimentary  sys- 
tem, the  tube  itself  acquiring  definite  form  simultaneously 
with  the  production  of  the  body  of  the  embryo. 

The  first  indication  of  the  alimentary  canal  appears  at  a 
very  early  period  of  development,  being  inaugurated  in  fact 
by  those  important  alterations  that  serve  to  differentiate  the 

168 


THE  DEVELOPMENT  OE  THE  DIGESTIVE  SYSTEM.   169 

blastodermic  vesicle  into  the  body  of  the  embryo  and  the 
embryonic  appendages.  It  will  be  remembered  that,  after 
the  splitting  of  the  parietal  plate  of  the  mesoderm  into  its 
two  lamellae,  and  the  union  of  the  outer  of  the  layers  with 
the  ectoderm  and  of  the  inner  with  the  entoderm  to  form 
respectively  the  somatopleure  and  the  splanchnopleure,  these 
two  double-layered  sheets  undergo  folding  in  different  direc- 
tions. Before  the  folding  occurs,  the  germ  is  a  hollow 
sphere  whose  cavity  is  the  archenteron  and  whose  walls 
are  the  somatopleure  and  the  splanchnopleure.^  A¥hile  the 
somatopleure  in  a  zone  corresponding  with  the  margin  of  the 
embryonic  area  becomes  depressed  and  is  carried  under  that 
area  to  form  the  lateral  and  ventral  body-wall  of  the  embryo 
(Plate  II.,  Figs.  2,  3,  and  4),  and  also  more  distally  folds 
up  over  the  area  to  produce  the  amnion  and  the  false  amnion, 
the  splanchnopleure,  likewise  in  a  line  corresponding  with 
the  periphery  of  the  embryonic  area,  is  depressed  and  carried 
inward  from  all  sides  toward  the  position  of  the  future 
umbilicus.  This  folding  in  of  the  splanchnopleure  effects 
the  division  of  the  archenteron  into  two  parts,  a  smaller 
cavity  falling  within  the  body  of  the  embryo,  which  latter 
is  forming  at  the  same  time,  and  a  larger  extra-embryonic 
compartment,  which  is  the  yolk-sac  or  umbilical  vesicle.  The 
intra-embryonic  cavity  is  the  gut-tract.  The  constricted 
communication  between  the  two  is  the  vitelline  duct.  While 
the  vitelline  duct  is  still  a  rather  wide  aperture,  the  anterior 
and  posterior  parts  of  its  intestinal  orifice  are  designated 
respectively  the  anterior  and  the  posterior  intestinal  portals. 

As  the  somatopleure  closes  in  around  the  vitelline  duct,  it 
forms  the  wall  of  the  abdomen,  the  opening  left,  which  is 
traversed  by  the  duct,  being  the  umbilical  aperture. 

It  is  evident  therefore  that  the  lining  of  the  gut-tract  is 
constituted  by  the  innermost  germ-layer,  the  entoderm,  and 
that  all  its  epithelial  elements  are  consequently  of  entodermic 
origin.  The  folding  in  of  the  splanchnopleure  begins  at 
about  the  end  of  the  second  week,  and  is  so  far  advanced 

^  Strictly  speaking,  the  somatopleure  and  the  splanchnopleure  are  not 
formed  before  the  folding  occurs,  but  the  processes  go  on  at  the  same  time. 


170  TEXT-BOOK  OF  EMBRYOLOGY. 

A  B 


Fig. 78.— Reconstructions  of  human  embryo  of  ulxml  .seventeen  days  (His) :  ov, 
optic  and  ot,  otic  vesicles  ;  nc,  nc',  notochord  ;  hdf/,  head-gut ;  g,  mid-gut ;  hg,  hind- 
gut;  t)«,  vitelline  sac;  Z,  liver;  v,  ta,  primitive  ventricle  and  truncus  arteriosus; 
va,  da,  ventral  and  dorsal  aortae;  aa,  aortic  arches;  jv,  primitive  jugular  vein;  cv, 
cardinal  vein ;  dC,  duct  of  Cuvier ;  uv,  ua,  umbilical  vein  and  artery ;  al,  allantois : 
uc,  umbilical  cord. 

before  the  end  of  the  third  week  that  the  archenteron  is  defi- 
nitely divided  into  the  gut-tract  and  the  yolk-sac. 


THE  DEVELOPMENT  OF  THE  DIGESTIVE  SYSTEM.   171 

In  its  earliest  definite  form,  then,  the  gut-tract  is  a  tube 
extending  from  one  end  of  the  embryonic  body  to  the  other, 
which  opens  widely  at  the  middle  of  its  ventral  aspect  into 
the  vitelline  duct,  but  which  is  closed  at  both  ends.  It  is 
usual  to  speak  of  the  primitive  gut-tract  as  consisting  mor- 
phologically of  three  parts,  the  head-gut,  which  is  the  region 
on  the  head  ward  side  of  the  orifice  of  the  vitelline  duct ; 
the  hind-gut,  which  is  the  part  near  the  tail-end  of  the 
embryo ;  and  the  mid-gut  or  intervening  third  portion 
(Fig.  78). 

The  closed  head-end  of  the  gut-tube  corresponds  with  the 
floor  of  the  primitive  mouth-cavity,  the  two  spaces  being 
separated  by  a  thin  veil  of  tissue,  which  consists  of  the 
entoderm  and  the  ectoderm  and  is  called  the  pharyngeal 
membrane  (Fig.  79).     A  considerable  proportion  of  the  so- 


FiG.  79.— Median  section  through  the  head  of  an  embryo  rabbit  6  mm.  long 
(after  Mihalkovics) :  rh,  membrane  between  stomodseum  and  fore-gut,  pharyngeal 
membrane  (Rachenhaut) ;  hp,  place  from  which  the  hypophysis  is  developed ;  h, 
heart;  M,  lumen  of  fore-gut;  ch,  chorda;  v,  ventricle  of  the  cerebrum;  v^,  third 
ventricle,  that  of  the  between-brain  (thalamencephalon) ;  «*,  fourth  ventricle,  that 
of  the  hind-brain  and  after-brain  (epencephalon  and  metencephalon,  or  medulla 
oblongata) ;  ck,  central  canal  of  the  spinal  cord. 

called  head-gut  constitutes  the  primitive  pharynx.  This 
region  of  the  tube  has  a  relatively  large  caliber,  and  pre- 
sents on  its  lateral  and  ventral  walls  the  series  of  recesses 
or  evaginations  known  as  the  throat-pockets  or  pharyngeal 
pouches  (Fig.  60). 


172 


TEXT-BOOK  OF  EMBRYOLOGY. 


While  the  inner,  entodermic  layer  of  the  gut-tube  becomes 
the  intestinal  mucosa,  the  outer,  mesodermic  stratum  produces 
the  muscular  and  the  connective-tissue  parts  of  the  bowel- 
wall,  the  most  superficial  layer  of  the  latter  with  its  meso- 
thelial  or  endothelial  cells  forming  the  visceral  layer  of  the 
peritoneum.  Since  the  mesodermic  layer  of  the  splanchno- 
pleure  of  each  side  is  continuous  with  the  corresponding 
mesodermic  layer  of  the  somatopleure  on  either  side  of  the 
embryonic  axis,  the  primitive  intestinal  canal  has  a  broad 
area  of  attachment  with  the  dorsal  wall  of  the  body-cavity 
(Fig.  80).     The  ventral  wall  is  likewise  connected  with  the 


Mesoderm. 


Fig.  80. — Transverse  section  of  a  sixteen-and-a-half-day  sheep-embryo  (Bonnet). 


ventral  body-wall  throughout  the  anterior  or  upper  part  of 
its  extent  by  the  continuity  of  the  splanchnopleuric  meso- 
derm of  each  side  with  the  somatopleuric  mesoderm  of  the 
same  side.  As  development  advances,  the  body-cavity  in- 
creases in  caliber  more  rapidly  than  does  the  intestinal  tube, 
so  that  the  interval  between  the  two  is  augmented,  in  conse- 
quence of  which  the  masses  of  connective  tissue  uniting  the 
dorsal  and  the  ventral  surfaces  of  the  gut  with  the  corre- 


THE  DEVELOPMENT  OF  THE  DIGESTIVE  SYSTEM.   173 

sponding  walls  of  the  body-cavity  become  drawn  out  so  as  to 
constitute  in  each  case  a  median  vertical  fold  consisting  of 
two  closely  approximated  layers  of  serous  membrane  with  a 
little  connective  tissue  between  them.  These  folds  are  the 
dorsal  and  the  ventral   mesenteries  (Fig.   81).      While  the 


A  ^  Q 

Fig.  81.— Diagrammatic  cross-sections  of  the  body  of  the  embryo  in  the  region 
of  the  heart  at  level  of  future  diaphragm  :  a,  esophageal  segment  of  gut-tract ;  b, 
dorsal  mesentery ;  c,  mesocardium  posterius  ;  d,  mesocardium  anterius  ;  e,  begin- 
ning of  septum  transversum,  containing  vitelline  and  allantoic  veins  ;  /,  septum 
transversum  ;  g,  thoracic  prolongation  of  abdominal  cavity  ;  nc,  neural  canal. 

dorsal  mesentery  extends  throughout  the  entire  length  of  the 
canal,  the  ventral  fold  is  present  only  at  its  anterior  or  upper 
part,  corresponding  in  the  extent  of  its  attachment  to  the 
digestive  tube  to  that  portion  representing  the  future  stomach 
and  upper  part  of  the  duodenum  (Fig.  82).  The  ventral 
mesentery  at  first  is  present  throughout  the  entire  extent  of 
the  canal,  but  very  early  undergoes  obliteration  except  in  the 
situation  above  noted.  Concerning  the  reason  and  the  method 
of  its  disappearance  nothing  is  definitely  known. 

The  intestinal  tube,  at  a  comparatively  early  stage,  pre- 
sents on  its  ventral  surface  near  the  posterior  or  caudal  end  a 
small  evagination  that  enlarges  to  form  the  allantois  (see  p. 
80).  While  a  part  of  the  intra-embryonic  portion  of  the 
allantois  dilates  and  develops  into  the  bladder,  the  part  be- 
tween this  latter  and  the  intestine  is  known  as  the  urogenital 
sinus.  The  part  of  the  gut-tube  posterior  to,  caudad  of,  the 
origin  of  the  allantois,  is  a  blind  pouch  known  as  the  cloaca. 
The  latter  is,  therefore,  the  common  termination  of  the  urinary 
and  the  intestinal  tracts. 

To  repeat,  we  have  now,  in  the  thii^d  week  of  development, 
the  alimentary  canal  represented  by  a  single  straight  tube 


174 


TEXT-BOOK  OF  EMBRYOLOGY. 


(compare  Fig.  78),  closed  at  each  end,  but  with  mouth-cavity 
and  anus  both  indicated,  the  tube  lying  within  a  larger  tube, 
the  body-cavity,  with  the  walls  of  which  latter  it  is  connected 
by  the  dorsal  and  ventral  mesenteries.    Along  the  dorsal  wall 


Fig.  82.— Reconstruction  of  human  embryo  of  about  seventeen  days  (after  His) : 
ov,  optic  and  ot,  otic  vesicles;  nc,  notochord;  hdg,  head-gut;  g,  mid-gut;  hg,  hind- 
gut;  vs,  vitelline  sac;  I,  liver;  v,  primitive  ventricle;  va,  da,  ventral  and  dorsal 
aortae;  jv,  primitive  jugular  vein;  cv,  cardinal  vein;  dC,  duct  of  Cuvier;  uv,  ua, 
umbilical  vein  and  artery  ;  al,  allantois ;  its,  umbilical  cord ;  dm,  dorsal  mesentery ; 
vm,  ventral  mesentery  (modiiied  from  His). 

of  the  body-cavity,  dorsad  to  the  parietal  peritoneum,  pass 
the  two  primitive  aortse,  and  later,  the  single  aorta  which 
results  from  the  fusion  of  these  two.  Between  the  two  folds 
of  the  dorsal  mesentery  pass  the  blood-vessels  that  nourish 
the  walls  of  the  gut.     Within  the  ventral  mesentery  are  the 


THE  MOUTH.  175 

vitelline  veins,  which  bring  the  blood  from  the  yolk-sac  and 
convey  it  to  the  primitive  heart.  On  the  ventral  wall  of  the 
gut  is  the  wide  aperture  of  the  vitelline  duct.  Farther 
caudad,  also  on  the  ventral  surface  of  the  bowel,  is  the  orifice 
of  the  allantois.  These  condition^  may  be  better  understood 
by  reference  to  Figs,  78  and  82.  Before  tracing  the  further 
development  of  the  abdominal  part  of  the  alimentary  system, 
it  will  be  proper  to  note  certain  very  important  processes 
pertaining  to  its  anterior  or  head-extremity,  and  also  to  con- 
sider the  formation  of  the  anus. 


THE  MOUTH. 

The  development  of  the  mouth,  the  tongue,  the  teeth,  and 
the  salivary  glands  has  been  fully  described  on  pages  122- 
131.  In  this  connection,  therefore,  it  will  be  necessary  to 
call  attention  to  only  a  few  of  the  salient  features  of  their 
evolution. 

The  oral  cavity  is  produced  by  a  folding  in  of  the  surface- 
ectoderm,  the  fossa  thus  formed  becoming  deeper  until  it 
meets  the  head-end  of  the  gut-tract.  From  the  walls  of  this 
fossa  the  salivary  glands  are  developed  as  evaginations,  in  the 
manner  already  described,  while  the  teeth  are  specialized 
growths  of  its  ectodermal  lining  and  of  the  underlying  meso- 
derm (vide  p.  125).  The  first  intimation  of  this  infolding 
is  apparent  at  the  twelfth  day  in  the  form  of  a  localized 
thickening  of  the  surface-cells  on  the  ventral  surface  of  the 
body  of  the  embryo  near  the  head-end.  The  thickened  area 
is  the  oral  plate,  which  speedily  becomes  depressed,  produc- 
ing the  oral  pit  or  fossa.  By  the  third  week,  the  oral  fossa 
or  stomodeum  is  a  well-marked  pit  of  pentagonal  outline,  its 
boundaries  being  the  nasofrontal  process  above,  the  maxillary 
processes  laterally,  and  the  mandibular  arches  below.  The 
original  oral  plate,  having  receded  farther  and  farther  from 
the  surface  and  forming  the  posterior  limit  of  the  mouth- 
cavity,  now  separates  that  cavity  from  the  pharyngeal  region 
of  the  gut-tube  and  comes  into  contact  with  the  anterior  wall 
of  the  latter.     It  is  called  the  pharyngeal  membrane  (Fig.  79). 


176  TEXT-BOOK  OF  EMBRYOLOGY. 

Its  disappearance  occurs  at  some  time  during  the  fourth  week, 
by  which  event  the  gut-tube  is  brought  into  communication 
with  the  mouth. 

The  exact  position  of  the  pharyngeal  membrane  is  not 
easily  definable.  It  is  certain,  however,  that  it  falls  farther 
back  than  the  posterior  limit  of  the  adult  oral  cavity, 
since  the  primitive  mouth  includes  the  anterior  part  of  the 
adult  pharynx.  For  example,  the  diverticulum  that  gives 
rise  to  the  anterior  lobe  of  the  pituitary  body  belongs  to 
the  primitive  mouth,  yet  its  vestige,  the  pharyngeal  bursa 
or  Rathk^'s  pocket,  is  found  in  the  pharynx  of  the  adult. 
The  primitive  oral  cavity,  by  the  growth  of  the  palate,  be- 
comes divided  into  the  adult  mouth  and  the  nasal  cavities. 
The  hard  palate  is  completed  in  the  ninth  week  and  the  soft 
palate  in  the  eleventh  week. 


THE  PHARYNX. 

The  pharynx  is  represented  in  the  embryo  by  the  expanded 
cephalic  end  of  the  primitive  gut-tract.  It  is  of  greater  rela- 
tive length  in  the  earlier  stages  of  development  than  later, 
including  as  it  does,  almost  half  the  length  of  the  gut-tube 
in  the  fourth  and  fifth  weeks.  The  primitive  pharyngeal 
cavity  is  widest  at  its  anterior  or  cephalic  extremity  and 
narrowest  at  the  opposite  end,  tapering  here  into  the  esoph- 
agus. Until  the  breaking  down  of  the  pharyngeal  mem- 
brane, which  takes  place  in  the  fourth  week,  this  structure 
marks  the  anterior  limit  of  the  pharynx  and  separates  it 
from  the  oral  cavity. 

The  pharyngeal  pouches  or  throat-pockets  have  been  re- 
ferred to  in  connection  with  the  visceral  arches  on  page  100. 
They  are  out-pocketings  or  evaginations  of  the  entodermal 
lining  of  the  pharynx,  there  being  four  furrows  on  each 
lateral  wall,  and  they  pass  from  the  ventral  toward  the 
dorsal  wall  of  the  cavity,  each  pouch  lying  between  two 
adjacent  visceral  arches.  The  entoderm  of  the  pouches 
comes  into  close  relation  with  the  ectoderm  of  the  outer 
visceral  furrows  (Fig.  60).     The  mesodermic  stratum  being 


THE  PHARYNX.  177 

absent  from  the  pharyngeal  pouches,  the  ectoderm  and  the 
entoderm  are  in  contact,  and  constitute  the  closing  membrane. 
As  previously  mentioned,  this  closing  membrane  ruptures  in 
aquatic  vertebrates,  in  consequence  of  which  the  pharyngeal 
cavity  in  such  animals  acquires  a  number  of  openings.  In 
man,  as  in  other  mammals,  and  in  birds,  such  rupture  prob- 
ably never  occurs.  Since  the  visceral  arches  and  clefts  are 
fully  considered  in  Chapter  VII.,  it  will  be  necessary  in  this 
connection  to  refer  only  to  such  derivatives  of  them  as  per- 
tain directly  to  the  pharynx. 

The  first  inner  cleft  or  pharyngeal  pouch  becomes  closed  oflP 
from  the  pharyngeal  cavity,  its  dorsal  end  giving  rise  to  the 
tympanic  cavity  or  middle  ear,  while  the  remaining  part  con- 
stitutes the  Eustachian  tube.  Hence,  the  tympanum  and  the 
Eustachian  tube  are  to  be  regarded  as  differentiated  portions 
of  the  primitive  pharyngeal  cavity.  The  dorsal  part  of  the 
closing  membrane  of  this  cleft  persists  as  the  tympanic  mem- 
brane. 

The  third  pharyngeal  pocket  or  third  inner  visceral  cleft, 
by  an  evagination  of  its  entodermal  epithelium,  gives  rise  to 
the  epithelial  parts  of  the  thymus  body,  the  connective- 
tissue  elements  of  this  "gland  "  being  furnished  by  the  meso- 
dermic  cells  which  surround  the  epithelial  diverticulum  and 
ultimately  enclose  and  isolate  its  branching  processes.  In  a 
similar  manner,  the  fourth  pocket  produces  the  lateral  lobes 
of  the  thyroid  body. 

The  ventral  wall  of  the  pharynx,  between  the  anterior 
extremities  of  the  second  throat-pockets,  evaginates  into  an 
entodermic  tube  which  extends  caudalward  to  develop  sub- 
sequently into  the  median  lobe  of  the  thyroid  body. 

The  tongue  (p.  131)  also  is  developed  from  the  walls 
of  the  pharynx,  the  anterior  unpaired  segment,  the  tuberculum 
impar,  growing  from  the  median  line  of  the  ventral  wall,  just 
below  the  level  of  the  first  visceral  arch,  while  the  two  sym- 
metrical segments  that  form  the  posterior  third  of  the  organ 
proceed  from  the  ventrolateral  walls  at  the  ventral  extremi- 
ties of  the  second  and  third  visceral  arches. 

The  tonsil  develops  as  masses  of  lymphoid  tissue  about 

12 


178 


TEXT-BOOK  OF  EMBRYOLOGY. 


an  evagination  of  the  lateral  wall  of  the  pharynx.     In  the 
third  month  the  lateral  pharyngeal  wall  pouches  out  to  form 

a  little  fossa  (Fig.  83,  1) 
which  is  situated  between  the 
second  and  third  visceral 
arches,  the  fossa  being  lined 
with  stratified  squamous  epi- 
thelium continuous  with  that 
of  the  pharyngeal  cavity. 
Little  solid  epithelial  buds 
(Fig.  83)  proceed  from  this 
diverticulum  into  the  sur- 
rounding connective  tissue, 
the  buds  subsequently  be- 
coming hollowed  out.  Wan- 
dering leukocytes  from  the 
neighboring  blood-vessels  in- 
filtrate the  connective  tissue 
around  the  young  crypts,  and  these  cells  becoming  aggre- 
gated into  condensed  and  isolated  groups  give  rise  to  the 
lymphoid  follicles  peculiar  to  the  tonsil.  The  separate  and 
well-differentiated  condition  of  the  follicles  is  not  attained 
until  some  months  after  birth.  TJie  place  of  origin  of  the 
tonsil  between  the  second  and  third  visceral  arches  explains 
the  position  of  the  adult, organ  between  the  anterior  and  pos- 
terior palatine  arches,  since  the  latter  structures  represent  the 
deep  extremities  of  the  former. 


Fig.  83.— Section  through  anlage  of 
tonsil  of  a  human  fetus  (Tourneux) :  1, 
tonsiUar  pit,  continuous  with  mouth- 
cavity  ;  2,  secondary  diverticula ;  3,  solid 
epithelial  buds ;  4,  striped  muscular  fiber. 


THE  ANUS. 

The  early  stages  of  the  development  of  the  anus  are  simi- 
lar to  those  of  the  mouth.  The  so-called  anal  membrane  is 
produced  by  the  growing  together  of  the  ectoderm  and  the 
entoderm,  the  mesoderm  being  crowded  aside.  The  site  of 
the  anal  membrane,  or  anal  plate,  is  in  the  median  line  of  the 
dorsal  surface  of  the  embryonic  body,  at  its  posterior  or 
caudal  extremity.  It  makes  its  appearance  in  the  third 
week.  Since  the  tissue  immediately  in  front — that  is,  head- 
ward,  of  the  anal  plate  projects  and  develops  into  the  primi- 


THE  ANUS. 


179 


tive  tail,  and  since  the  axis  of  the  body  becomes  ventrally 
curved,  the  anal  plate  is  carried  around  somewhat  toward 
the  ventral  aspect  of  the  body.  During  the  following  fort- 
night the  anal  plate  becomes  depressed  so  as  to  form  a  small 
fossa,  which  is  often  designated  the  anal  pit  or  proctodeum. 
The  position  of  the  anal  pit  does  not  correspond,  in  any 
vertebrate,  to  the  end  of  the  intestine,  but  to  a  point  short 
of  it ;  the  gut,  therefore,  extends  beyond  the  position  of  the 
anus.  This  portion  of  the  bowel  is  the  postanal  gut  of  ver- 
tebrate morphology.     Ultimately  it  entirely  disappears. 

While  the  anal  pit  is  forming,  the  allantois  is  growing 
forth  as  a  diverticulum  from  the  ventral  wall  of  the  gut 
(Fig.  84).      The   intra-embryonic   part  of  the   allantois   is 


Fig.  84.— Sagittal  section  of  caudal  extremity  of  cat  embryo  of  6  mm. :  1,  cloaca ; 
2,  cloacal  membrane ;  3,  intestine ;  4,  post-anal  gut ;  5,  allantoic  canal ;  6,  chorda 
dorsalis ;  7,  medullary  canal  (Tourneux). 


transformed  chiefly  into  the  urinary  bladder,  but  it  gives 
rise  also,  by  its  proximal  extremity,  to  a  short  wide  duct, 
the  urogenital  sinus,  which  is  an  avenue  of  communication 
with  the  bowel.  The  part  of  the  gut  on  the  caudal  side  of 
the  aperture  of  the  urogenital  sinus  is  the  cloaca,  which  is 
the  common  termination,  therefore,  of  the  genito-urinary 
system  and  of  the  intestinal  canal. 

The  surface  depression  referred  to  above  as  the  anal  pit  is 


180  TEXT-BOOK  OF  EMBBYOLOGF. 

often  called  the  cloacal  depression  during  the  time  that  the 
cloaca  is  present.  In  the  lowest  mammals,  the  monotremes, 
as  also  in  the  Amphibia,  in  reptiles,  and  in  birds,  the  cloaca 
is  a  permanent  structure.  By  the  breaking  down  of  the  mem- 
brane between  it  and  the  cloacal  depression,  it  acquires  an  out- 
let, through  which  the  feces,  the  urine,  and  the  genital  products 
find  egress.  In  all  higher  mammals,  however,  including  man, 
the  cloaca  suffers  division  into  an  anterior  or  ventral  pass- 
age-way, the  urogenital  sinus,  and  a  posterior  canal,  the  rec- 
tum and  canal  of  the  anus.  This  division  is  effected  by  the 
growth  of  three  ridges  or  folds,  of  which  one  grows  from  the 
point  of  union  of  the  urogenital  sinus  and  the  gut,  while  the 
other  two  proceed,  one  from  each  lateral  wall  of  the  cloaca. 
The  three  folds  coalesce  to  form  a  perfect  septum.  The 
division  is  complete  at  about  the  end  of  the  second  month 
(or,  according  to  Minot,  at  the  fourteenth  week).  The 
cloacal  depression  or  anal  pit  shares  in  this  division,  so  that 
at  about  the  tenth  week,  it  is  separated  into  the  anal  pit 
proper,  or  the  proctodeum,  and  the  orifice  of  the  urogenital 
sinus.  The  newly-formed  septum  continues  to  thicken, 
especially  near  the  surface  of  the  body,  until  it  constitutes 
the  pyramidal  mass  of  tissue  known  as'  the  perineal  body,  or 
perineum. 

.  The  anal  pit  deepens,  the  anal  membrane  being  thereby 
approximated  to  the  end  of  the  bowel,  and  in  the  fourth 
month  the  anal  membrane  breaks  down  and  disappears. 
Persistence  of  the  anal  membrane  after  birth  constitutes  the 
anomaly  known  as  imperforate  anus. 

THE  DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL 
INTO  SEPARATE   REGIONS. 

The  fourth  week  marks  the  beginning  of  certain  impor- 
tant changes  in  the  simple  straight  alimentary  tube.  The 
reader  is  again  reminded  that  this  tube  is  connected  with 
the  dorsal  body-wall  by  the  dorsal  mesentery  and  with  the 
ventral  wall,  for  a  part  of  its  extent,  by  the  ventral  mesen- 
tery ;  that  the  canal  is,  as  yet,  without  communication  with 
the  exterior;  and  also  that  the  vitelline  duct  and  the  allan- 


DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL.   181 

tois  are  connected  with  its  ventral  surface  (Fig.  82).  Tlie 
umbilical  vesicle  having  reached  the  limit  of  its  development 
in  the  fourth  week  and  having  begun  to  shrink,  the  vitelline 
duct  likewise  begins  to  retrograde  and  very  soon  becomes 
an  inconspicuous  structure. 

The  dorsal  wall  of  the  tube  at  a  point  nearer  the  head-end 
begins  to  bulge  toward  the  dorsal  body- wall,  forming  a  some- 
what spindle-shaped  enlargement  (Figs.  85,  86).     This  di- 


Middle  lobe 

of  thyroid  gland. 

Thymus  gland. 
Latei'al  lobe 

of  thyroid  gland. 

Trachea. 
Lung. 


Right  lobe  of  liver. 


Vitelline  duct. 


Pharyngeal 
pouches. 


Stomach. 

Pancreas. 

Left  lobe  of  liver. 


Small  intestine. 


Lar^e  intestine. 


Fig.  85. — Scheme  of  the  alimentary  canal  and  its  accessory  organs  (Bonnet). 

lata ti on  is  the  beginning  of  the  future  stomach.  The  part 
of  the  canal  on  the  cephalic  side  of  the  stomach  lags  behind 
somewhat  in  growth,  corresponding  in  this  respect  with  the 
relatively  smaller  size  of  the  adult  esophagus.  The  esoph- 
agus begins  to  lengthen  in  the  fourth  week.  At  this  time, 
also,  the  beginning  of  the  liver  is  indicated  by  a  small  diver- 


182  TEXT-BOOK  OF  EMBRYOLOGY. 

ticulum  which  pouches  out  from  the  ventral  wall  of  the 
intestine  just  posterior  to  (below)  the  stomach — the  future 
duodenal  region  therefore — and  which  grows  into  the  ventral 
mesentery.  Very  soon  after  the  appearance  of  the  hepatic 
evagination,  a  similar  out-pouching  from  the  dorsal  wall  of 


Fig.  86.— Outline  of  alimentary  capal  of  human  embryo  of  twenty-eight  days 
(His) :  ph,  pituitary  fossa  ;  tg,  tongue  ;  Ix,  primitive  larynx ;  o,  esophagus ;  tr,  trachea ; 
Ig,  lung ;  s,  stomach  ;  p,  pancreas  ;  hd,  hepatic  duct ;  vd,  vitelline  duct ;  al,  allantois ; 
hg,  hind-gut ;  Wd,  Wolffian  duct ;  k,  kidney. 

the  future  duodenal  region  of  the   intestine   indicates  the 
beginning  of  the  development  of  the  pancreas. 

In  the  latter  part  of  the  third  week  or  in  the  beginning  of 
the  fourth,  the  esophagus  presents  a  longitudinal  groove  on 
the  inner  face  of  its  ventral  wall.  This  groove  increases  in 
depth  and  caliber  and  finally  becomes  constricted  oif  from 
the  esophagus,  with  which  it  retains  connection  only  at  its 
pharyngeal  end.  The  tube  or  tubular  sac  thus  formed  is  the 
first  step  in  the  development  of  the  lungs  and  the  trachea. 


DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL.   183 

It  may  be  said  then  that  the  gut-tract  has  now,  in  the 
fourth  week,  reached  the  stage  of  differentiation  into  the  phar- 
ynx, the  esophagus,  the  stomach,  and  the  intestine,  with  the 
liver,  the  pancreas,  the  respiratory  system,  and  the  allantois 
fairly  begun. 

As  heretofore  pointed  out  (p.  81),  the  allantois — which 
grows  directly  from  the  primitive  gut-tract,  and  which  con- 


FiG.  87.— Outline  of  alimentary  canal  of  human  embryo  of  thirty-five  days 
(His) :  pb,  pituitary  fossa ;  tg,  tongue ;  Ix,  primitive  larynx ;  o,  esophagus ;  tr, 
trachea ;  Ig,  lung ;  s,  stomach ;  p,  pancreas ;  hd,  hepatic  duct ;  c,  csecum ;  d,  cloaca ; 
fc,  kidney ;  a,  anus  ;  gp,  genital  eminence  ;  t,  caudal  process. 

sists  therefore  of  the  entoderm  and  the  visceral  mesoderm — 
although  destined  to  produce  in  part  the  permanent  bladder, 
functionates  for  a  time,  after  its  union  with  the  false  amnion 
to  form  the  chorion,  as  an  organ  of  respiration ;  while  the 
permanent  respiratory  system,  as  we  have  seen,  likewise 
develops  from  the  entodermal  epithelium  of  the  gut- 
tract.     The  entoderm,  therefore,  sustains  an  important  re- 


184  TEXT-BOOK  OF  EMBRYOLOGY. 

lation  to  the  nutrition  of  both  the  embryonic  and  the  adult 
organism. 
Increase  in  lyength  and  Further  Subdivision. — 

The  intestinal  canal  grows  in  length  much  more  rapidly  than 
does  the  embryonic  body.  It  is  in  consequence  of  this  dis- 
proportionate growth  that  the  tube  becomes  bent  and  thrown 
into  coils  or  convolutions.  During  the  fifth  and  sixth  weeks 
a  conspicuous  flexure  appears  at  some  distance  below  the 
stomach.  Here  the  bowel  assumes  the  form  of  a  U-shaped 
tube,  the  closed  end  of  the  U  projecting  toward  the  ventral 
body-wall  (Fig.  88).     In  other  words,  the  redundant  portion 


Stomach.  ^ 

Lesser  curve  '  ^ ~  Mesogastrium. 

of  slotnach. | 

Greater  curve _'  | — Spleen. 


of  stomach 


^Pancreas. 


Mesentery. 

CcBcum.^^  h.    y Rectum. 

Fig.  88.— Intestinal  canal  of  human  embryo  of  six  weeks  (Toldt). 

of  the  gut  is  pulled  away,  as  it  were,  from  the  dorsal  wall  of 
the  body-cavity  and,  as  a  consequence,  the  dorsal  mesentery 
is  lengthened  in  this  region  to  a  corresponding  extent  (Fig.  88). 
The  vitelline  duct  is  attached  to  the  part  of  the  bend  near- 
est the  ventral  wall  (Fig.  86).  At  a  point  on  the  lower  limb 
of  the  U  the  bowel  abruptly  acquires  increased  caliber.  This 
dilated  part  is  the  beginning  of  the  caecum  or  head  of  the 
colon,  and  its  appearance  initiates  the  distinction  between  the 
large  and  the  small  intestine,  since  the  part  of  the  bowel  on 
the  distal  side  of  the  point  in  question  becomes  also  of  larger 
caliber  and  forms  the  colon. 

During  the  succeeding  week  or  fortnight,  the  character  of 
the  colon  and  of  the  csecum  becomes  better  established.  The 
remaining  part  of  the  lower  limb  of  the  U-loop,  with  all  of 


DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL.   185 


the  tube  included  between  the  loop  and  the  stomach,  is  the 
small  intestine,  which  presents  a  slight  dorsal  flexure  at  its 
proximal  extremity.  The  stomach  meanwhile  has  increased 
in  size  and  has  almost  attained  its  characteristic  shape.  By 
the  end  of  the  sixth  week,  then,  the  alimentary  canal  has  not 
only  increased  in  length  but  has  so  far  differentiated  as  to 
have  acquired  stomach,  duodenum,  small  intestine,  caecum,  and 
rectum. 

Alteration  in  the  Relative  Position  of  Parts,  and 
Further  Development. — The  most  important  modification 
of  the  alimentary  tube  as  it  exists  at  the  end  of  the  sixth 
week  is  effected  by  certain  changes  of  position  of  some  of  its 
parts.  The  stomach  and  the  large  intestine  are  the  portions 
of  the  tract  most  conspicuously  affected.  The  lower  limb  of 
the  U-segment  of  bowel,  which  consists  chiefly  of  the  rudi- 
mentary caecum  and  a  part  of  the  colon,  is  lifted,  as  it  were, 
over  the  upper  limb  and  comes  to  occupy  a  position  above  it 
(Fig.  89,  A),  the  caecum  assuming  a  position   in  the  right 


:b  c 

Fig.  89.— Three  successive  stages  showing  the  development  of  the  digestive 
tube  and  the  mesenteries  in  the  human  fetus  (modified  from  Tourneux) :  1,  stom- 
ach ;  2,  duodenum ;  3,  small  intestine  ;  4,  colon ;  5,  vitelline  duct ;  6,  caecum ;  7,  great 
omentum;  8,  mesoduodenum ;  9,  mesentery;  10,  mesocolon.  The  arrow  points  to 
the  orifice  of  the  omental  bursa.    The  ventral  mesentery  is  not  shown. 

hypochondriac  region,  and  the  colon  passing  thence  trans- 
versely across  the  abdomen  ventrad  to  the  duodenum. 
This  shifting  of  position  on  the  part  of  the  colon  brings 


186  TEXT-BOOK  OF  EMBRYOLOGY. 

about  important  complications  in  the  arrangement  of  the  mes- 
entery, since  the  part  of  the  dorsal  mesentery  that  pertains  to 
the  upper  part  of  the  colon  correspondingly  alters  its  position 
and  line  of  attachment,  becoming  adherent  to  the  peritoneum 
on  the  ventral  surface  of  the  duodenum.  The  part  of  the 
mesentery  in  question  becomes  the  transverse  mesocolon 
(Fig.  89,  B).  The  large  intestine,  after  this  change  of  posi- 
tion, presents  caecum,  transverse  colon,  descending  colon,  and 
rectum,  the  ascending  colon  being  still  absent. 

The  vermiform  appendix  in  the  third  month  has  already 
acquired  the  form  of  a  slender  curved  tube  projecting  from 
the  caecum.  At  the  time  of  its  first  appearance  and  for  some 
weeks  afterward,  the  appendix  has  the  same  caliber  as  the 
caecum.  Subsequently  the  csecum  outstrips  the  appendix  in 
growth,  the  latter  appearing  in  the  adult  state  as  a  relatively 
very  small  tube  attached  to  the  much  larger  caecum. 

The  caecum  soon  begins  again  to  change  its  position,  gradu- 
ally moving  downward  toward  the  right  iliac  fossa  (Fig.  89). 
The  downward  migration  of  the  caecum  is  due  to  the  growth 
of  the  colon  in  the  same  direction.  In  this  manner  the  ascend- 
ing colon  is  gradually  produced,  it  having  developed  to  such  an 
extent  in  the  seventh  montli  that  the  caecum  lies  below  the 
right  kidney,  while  in  the  eighth  month  it  passes  the  crest  of 
the  ilium.  Corresponding  with  the  growth  of  the  ascending 
colon,  the  mesentery  shifts  its  parietal  attachment  and  in- 
creases in  extent  until  the  ascending  mesocolon  is  produced ; 
and  with  the  descent  of  the  caecum,  the  terminal  part  of  the 
small  intestine  necessarily  alters  its  position  to  a  like  degree. 

The  stomach,  up  to  the  third  month,  is  a  localized  dilata- 
tion of  the  intestinal  tube,  bulging  most  in  the  dorsal  direc- 
tion and  having  its  long  axis  parallel  with  that  of  the  body 
(Fig.  88).  In  the  third  month,  however,  it  undergoes  an 
important  alteration  in  position,  rotating  about  two  axes. 
First,  it  turns  about  a  longitudinal  axis,  whereby  the  left 
side  comes  to  face  toward  the  ventral  surface  of  the  body 
(anteriorly)  and  the  right  surface  looks  toward  the  spinal 
column.  In  addition  to  the  longitudinal  rotation,  the  stom- 
ach also  rotates  upon  a  dorsoventral  (anteroposterior)  axis, 


DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL.   187 

by  which  the  lower  or  pyloric  extremity  moves  somewhat 
upward  and  to  the  right,  and  the  cardiac  end  goes  tail  ward 
(downward)  and  to  the  left  (Fig.  89).  By  this  double  rota- 
tion the  stomach  is  made  to  assume  approximately  its  adult 
position.  The  longitudinal  rotation  of  the  stomach,  in  which 
the  lower  portion  of  the  esophagus  takes  part,  explains  the 
relation  of  the  vagus  nerves  in  the  adult.  The  nerves,  before 
the  rotation,  lie  one  on  each  side  of  the  esophagus  and  stom- 
ach, but  since  the  left  surfaces  of  both  turn  forward  and  the 
right  surfaces  turn  backward,  the  left  vagus  lies  on  the 
anterior  surface  of  the  esophagus  and  of  the  stomach,  while 
the  right  nerve  is  in  relation  with  their  posterior  surfaces. 

The  relations  of  the  mesogastrium  are  influenced  in  an  im- 
portant manner  by  the  rotation  of  the  stomach.  As  long  as 
the  stomach  retains  its  original  position  and  relations,  with 
its  greater  curvature  facing  dorsad  (or  posteriorly),  the  meso- 
gastrium is  a  vertical  mesial  fold  of  peritoneum  (Fig.  88), 
while  the  ventral  mesentery  similarly  connects  the  future 
lesser  curvature  or  ventral  surface  of  the  stomach  with  the 
ventral  body-wall.  At  the  very  beginning  of  the  process  of 
rotation,  the  mesogaster  becomes  somewhat  redundant  and 
sags  toward  the  left  (Fig.  89,  A).  As  this  increases  in 
extent,  there  is  formed,  between  the  stomach  and  the  dorsal 
body-wall,  a  pouch  or  pocket,  the  omental  bursa,  whose  open- 
ing is  toward  the  right  (Fig.  89).  In  the  third  and  fourth 
months  the  original  mesogaster,  lengthening  more  and  more, 
and  being  affected  by  the  increasing  torsion  of  the  stomach, 
projects  in  the  form  of  a  sac  considerably  below  the  level  of 
the  stomach,  in  front  of  (ventral  to)  the  small  intestine  and 
the  transverse  colon.  It  ultimately  becomes  the  great  omen- 
tum. The  mesogastrium,  from  having  been  a  vertical  mesial 
fold,  is  now  become  a  transverse  fold,  so  redundant  as  to  be 
folded  upon  itself  and  to  constitute  a  bag. 

In  like  manner  the  ventral  mesentery  (Figs.  82  and  90), 
which  connects  the  anterior  or  ventral  surface  of  the  stomach 
with  the  ventral  body-wall,  and  in  which  the  liver  develops, 
is  altered  from  a  mesial  fold  to  a  transverse  fold  by  the  rota- 
tion of  the  stomach.     As  the  liver  migrates  to  a  position 


188  TEXT-BOOK  OF  EMBRYOLOGY. 

above  the  stomach,  the  part  of  the  ventral  mesentery  which 
connects  the  liver  with  the  body-wall  becomes  its  falciform 
ligament  and  coronary  ligament,  while  that  portion  of  this 
mesentery  that  connects  the  originally  ventral  surface  of  the 
stomach,  now  its  lesser  curvature,  with  the  liver  is  the  lesser 
or  gastrohepatic  omentum.  The  lesser  omentum,  therefore,  is 
the  anterior  or  ventral  boundary  of  the  orifice  of  the  omental 
bursa  referred  to  above. 

The  small  intestine  begins  to  exhibit  flexures  as  early  as 
the  fifth  week,  and  by  the  end  of  the  sixth  week  the  duo- 
denum is  well  indicated  as  a  segment  of  the  gut-tube  passing 
from  the  pyloric  end  of  the  stomach  toward  the  dorsal  body- 
wall.  From  this  time  the  development  of  the  small  intes- 
tine, aside  from  its  histological  characters,  consists  chiefly  in 
increase  in  length  with  consequent  modification  of  its  mesen- 
tery. A  striking  feature  of  human  development  is  that, 
with  the  growth  in  length  of  the  small  bowel,  it  is  gradually 
extruded  from  the  abdominal  cavity  into  the  tissues  of  the 
umbilical  cord.  The  extent  to  which  extrusion  takes  place 
increases  until  the  tenth  week,  after  which  period  the  intes- 
tine is  gradually  withdrawn  into  the  abdomen.  In  the 
fourth  month  it  lies  entirely  within  the  abdominal  cavity. 
Failure  of  complete  restoration  of  the  gut  to  the  cavity  of 
the  abdomen  constitutes  congenital  umbilical  hernia. 

The  histological  alterations  incident  to  the  develop- 
ment of  the  alimentary  tube,  from  the  beginning  of  the 
esophagus  to  the  end  of  the  rectum,  consist  in  the  differen- 
tiation of  the  constituent  elements  of  its  walls  from  the  two 
strata,  the  entoderm  and  the  visceral  mesoderm,  which  com- 
pose the  walls  of  the  early  gut-tube.  As  an  initial  step  in 
the  process,  the  cells  of  the  mesodermic  stratum  undergo  mul- 
tiplication and  arrange  themselves  in  a  narrow  loose  inner 
zone  and  a  thicker  outer  lamella.  The  inner  layer  subse- 
quently becomes  the  submucosa  of  the  fully  formed  state, 
while  the  cells  of  the  outer  layer  undergo  differentiation  into 
unstriped  muscular  tissue,  and  constitute  the  muscular  coat 
of  the  canal.  In  the  case  of  the  esophagus  and  stomach,  at 
least,  this  muscular  tunic,  in  the  fourth  month,  exhibits  the 


DIFFERENTIATION  OF  THE  ALIMENTARY  CANAL.   189 

distinction  between  inner  circular,  and  outer  longitudinal, 
layers.  The  surface-cells  of  the  mesodermic  stratum  of  the 
primitive  stomach  and  bowel  become  the  endotlielium  of  the 
serous  coat. 

The  glands  of  the  entire  canal  are  products  of  the  inner, 
entodermic  stratum,  and  therefore  they  are  intimately  related 
genetically,  as  well  as  histologically,  with  the  mucous  mem- 
brane. 

The  glands  of  the  stomach,  both  the  peptic  and  the  pyloric, 
originate  from  small  cylindrical  cell-masses  that  have  been 
produced  by  local  multiplication  and  aggregation  of  ento- 
dermal  cells.  By  the  hollowing  out  of  the  cylinders  and  the 
branching  of  the  tubes  thereby  formed,  the  two  varieties  of 
gastric  glands  are  evolved.  Both  sets  make  their  appearance 
in  the  tenth  week.  Until  the  fourth  month  the  peptic  glands 
contain  cells  of  but  one  type ;  at  this  period,  however,  cer- 
tain cells  of  these  glands  become  altered  by  the  gradual 
accumulation  of  granules  within  their  protoplasm,  by  which 
they  are  transformed  into  the  characteristic  acid  or  parietal 
cells  of  these  glands. 

The  glands  and  villi  of  the  intestine  are  likewise  products 
of  the  entodermal  lining  of  the  gut.  Their  evolution  begins 
in  the  second  month,  and  they  are  fairly  well  formed  by  the 
tenth  week.  As  in  the  case  of  the  gastric  glands,  the  glands 
of  the  bowel  develop  from  cylindrical  masses  of  entodermal 
cells  which  are  at  first  solid,  but  which  later  become  hollowed 
out  to  form  tubular  depressions  or  follicles.  In  the  region 
corresponding  to  the  upper  part  of  the  small  intestine  many 
of  these  follicles  branch  to  give  rise  to  the  glands  of  Brunner, 
while  unbranched,  simple,  tubular  depressions  distributed 
throughout  the  entire  length  of  the  bowel  become  the  glands 
of  LieberkUhn.  While  the  surface  entoderm  is  thus  growing 
into  the  underlying  mesodermic  tissue  to  form  the  glands,  it 
becomes  elevated  into  minute  projections  between  the  mouths 
of  the  gland-ducts,  forming  the  villi  of  the  intestinal  mucosa. 
The  connective-tissue  core  of  the  villus  is  derived  from  the 
underlying  mesodermic  tissue,  the  cells  of  which,  proliferat- 
ing, grow  forth  into  the  entoderm.     The  villi  at  first  are 


190  TEXT-BOOK  OF  EMBRYOLOGY. 

present  throughout  the  large  and  the  small  intestine  alike, 
being  well  developed  by  the  fourth  month.  While  the  villi 
of  the  small  bowel  continue  their  development,  those  of  the 
large  intestine,  after  the  fourth  month,  begin  to  retrograde. 
At  the  time  of  birth  they  are  still  discernible,  but  at  the  end 
of  the  first  month  after  birth  they  are  completely  obliterated. 
Meckel's  Diverticulum. — The  vitelline  duct,  it  will 
be  remembered,  is  the  avenue  of  communication  between 
the  early  gut-tube  and  the  umbilical  vesicle.  In  the  sixth 
week  the  umbilical  vesicle  has  already  begun  to  retrograde, 
and  the  vitelline  duct  is  attached  to  the  ventral  extremity 
of  the  U-loop  of  the  bowel  present  at  this  stage.  The  vitel- 
line duct  in  most  cases  suifers  complete  obliteration  in  the 
later  stages  of  fetal  life.  In  some  instances,  however,  its 
proximal  extremity  persists  in  the  form  of  a  small  blind  tube 
varying  in  length  from  one  to  several  inches,  which  is  known 
as  Meckel's  diverticulum.  Since  the  site  of  attachment  of  the 
vitelline  duct  is  not  far  from  the  termination  of  the  small 
intestine,  Meckel's  diverticulum,  when  present,  is  connected 
with  the  lower  part  of  the  ileum,  at  a  point  from  one  to  three 
feet  from  its  termination.  Should  this  tube  remain  attached 
to  the  umbilical  aperture  and  retain  a  patulous  orifice,  there 
would  result  a  congenital  fecal  fistula.^ 

THE  DEVELOPMENT  OF  THE  LIVER. 

The  essential  features  of  the  development  of  the  liver  will 
be  more  easily  apprehended  if  the  reader  will  not  lose  sight 
of  the  fact  that  the  organ  is  a  compound  tubular  gland,  and 
if,  further,  he  will  recall  the  method  by  which  glands  in 
general  are  developed — that  is,  as  evaginations  of  the  wall 
of  the  cavity  or  organ  to  which  they  pertain. 

The  first  step  in  the  evolution  of  the  liver  is  the  growth 
of  a  diverticulum  from  the  ventral  wall  of  the  gut-tube  at  a 
point  corresponding  to  the  region  of  the  future  duodenum. 
This  occurs  in  the  third  week,  since  His  foiuid  the  diver- 

^  Meckel's  diverticulum  is  of  interest  clinically,  since  by  contracting 
adhesions  to  adjacent  coils  of  intestine  or  by  entanglement,  it  may  produce 
acute  obstruction  of  the  bowel. 


THE  DEVELOPMENT  OF  THE  LIVER. 


191 


ticnlum  in  a  human  embryo  of  3  mm.  The  single  diver- 
ticulum ^  very  speedily  bifurcates  at  its  distal  extremity  (Fig. 
85).  The  very  short  time  that  elapses  between  the  first 
appearance  of  the  evagination  and  its  division  into  two 
branches  explains  the  statement  made  in  some  text-books 
that  two  diverticula  are  present  from  the  first.  The  hepatic 
diverticulum  is  said  to  grow  into  the  septum  transversum 
{vide  Development  of  the  Diaphragm,  p.  158).  The  dorsal 
part  of  the  septum  transversum  or  primitive  diaphragm,  the 
region  just  ventral  to  the  bowel,  contains  a  mass  of  young 
connective  tissue,  rich  in  cells  and  blood-vessels,  which  has 
been  designated  the  prehepaticus,  and  the  liver-ridge,  by  His 
and  Kolliker  respectively  (Fig.  78).  It  is  into  this  vascular 
and  cellular  mass  that  the  liver 
diverticulum  inserts  itself.  The 
septum  transversum  is  united  in 
the  median  plane  of  the  body  with 
the  ventral  mesentery,  and  since 
the  ventral  mesentery  is  connected 
withtheregion  oftheintestinefrom 
which  the  hepatic  diverticulum 
is  evaginated,  the  latter  passes 
between  the  two  layers  of  the 
mesentery  to  reach  the  liver- 
ridge  (Fig.  90).  This  fact  con- 
stitutes the  key  to  the  topo- 
graphical relations  of  the  liver 
and  its  peritoneal  "ligaments," 
as  will  appear  hereafter. 

The  two  diverticula  resulting 
from  the  division  of  the  original 
single  evagination  embrace  be- 
tween them  the  two  vitelline 
veins,  and  by  repeated  branch- 
ing produce  the   right  and  the 

left  lobes  of  the   liver.     Before  branching,  the  diverticula 
become    greatly   thickened   at   their   distal    extremities   by 

^  Single,  according  to  His,  Kolliker,  Hertwig,  Minot,  and  Piersol. 


Fig.  90.— Diagram  to  show  the 
original  positions  of  the  liver, 
stomach,  duodenum,  pancreas,  and 
spleen,  and  tlie  ligamentous  appa- 
ratus pertaining  to  them.  Tlie 
organs  are  seen  in  longitudinal 
section  :  I,  liver ;  to,  spleen ;  p,  pan- 
creas ;  dd,  small  intestine ;  dg,  vitel- 
line duct ;  hid,  caecum ;  md,  rectum ; 
kc,  lesser  curvature ;  gc,  greater 
curvature  of  the  stomach ;  mes, 
mesentery;  kn,  lesser  omentum 
(lig.  hepatogastricum  and»  hepato- 
duodenale) ;  Is,  ligamentum  sus- 
pensorium  hepatis  (Hertwig). 


192  TEXT-BOOK  OF  EMBRYOLOGY. 

abundant  cell-proliferation.  The  numerous  branches  into 
which  they  divide  are  not  tubes,  but  solid  cylinders  of. 
cells,  the  hepatic  cylinders.  The  secondary  branches  of 
these  cylinders  unite  with  corresponding  branches  of 
adjacent  systems,  producing  thereby  a  network  of  inoscu- 
lating cell-cords,  the  meshes  of  which  are  occupied  by 
young  connective-tissue  cells  and  the  developing  blood- 
vessels. The  connective  and  vascular  tissue  of  the  liver- 
ridge,  thus  surrounding  and  permeating  the  epithelial  cell- 
cords,  produces  all  the  connective-tissue  parts  of  the  liver, 
while  the  liver  parenchyma — the  proper  hepatic  cells — and 
the  epithelium  of  the  bile-ducts  originate  from  the  primitive 
entodermic  evagination.  The  cords  of  cells  are  in  part  hol- 
lowed out  to  form  the  bile-ducts  and  bile-capillaries,  and  in 
part  become  the  cells  of  the  lobules.  The  cylinders  that  are 
to  produce  the  bile-ducts  acquire  their  lumen  by  the  fourth 
week. 

Until  the  middle  of  the  fourth  month,  the  right  and  left 
lobes  of  the  liver  are  of  equal  size,  but  after  this  period 
the  right  lobe  outstrips  the  left  in  growth.  The  liver  grows 
very  rapidly  and  is  relatively  of  much  greater  size  in  the 
fetus  than  in  the  adult,  almost  filling  the  body-cavity  at  the 
third  month.  In  the  later  months  of  pregnancy  it  reaches 
almost  to  the  umbilicus,  while  at  birth  it  makes  up  one- 
eighteenth  of  the  body-weight. 

The  gall-bladder  develops  as  an  evagination  from  the 
original  diverticulum.  It  is  present  in  the  second  month. 
The  pedicle  of  this  evagination  lengthens  somewhat  and 
becomes  the  cystic  duct.  The  stalk  of  the  hepatic  evagina- 
tion itself  becomes  the  ductus  communis  choledochus. 

The  ligaments  of  the  liver,  save  the  round  ligament, 
are  simply  folds  of  the  peritoneum  which  connect  the  organ 
with  the  abdominal  wall.  Falling  into  the  same  category, 
though  not  usually  designated  a  ligament,  is  the  gastrohepatic 
omentum,  which  connects  the  liver  with  the  stomach.  These 
various  peritoneal  folds  may  be  looked  upon  as  parts  of  the 
ventral  mesentery.  Since  the  liver  evagination  grows  be- 
tween the  two  layers  of  the  ventral  mesentery  to  reach  the 


THE  DEVELOPMENT  OF  THE  LIVER.  -  193 

septum  transversum,  the  liver  will  be  found,  in  the  early 
stages  of  its  development,  embedded  between  the  lamellae 
of  this  mesentery,  which  is  a  median  vertical  fold  of  peri- 
toneum (Fig.  90).  The  liver  is  therefore  enclosed  in  the 
peritoneum  and  is  connected  below,  by  a  part  of  the  ventral 
mesentery,  with  the  lesser  curvature  of  the  stomach,  which 
still  lies  in  the  median  plane  of  the  body,  and  above  and  in 
front,  with  the  diaphragm  and  the  ventral  body- wall  by  the 
upper  and  anterior  part  of  the  same  structure.  The  latter 
fold  is  somewhat  modified  by  the  intimate  association  of  the 
early  stage  of  the  liver  with  the  primitive  diaphragm,  the 
liver  having  developed  within  a  portion  of  the  septum  trans- 
versum, the  liver  ridge.  As  development  advances,  a  par- 
tial separation  of  the  liver  and  the  diaphragm  is  effected,  the 
peritoneum,  as  it  were,  growing  between  the  two  from  both 
the  ventral  and  the  dorsal  edges  of  the  liver.  The  region 
which  is  not  invaded  by  the  peritoneum  represents  the  non- 
peritoneal  surface  of  the  adult  liver  between  the  lines  of  re- 
flection of  the  two  layers  of  the  coronary  ligament.  Since  the 
peritoneum  on  the  under  surface  of  the  diaphragm  is  reflected 
from  that  muscle,  both  in  front  of  and  behind  this  area  of 
contact,  to  become  continuous  with  the  peritoneum  on  the 
convex  surface  of  the  liver,  there  are  formed  two  transverse, 
parallel,  but  separated,  folds  which  constitute  the  coronary 
ligament  of  adult  anatomy.  The  lateral  prolongations  of 
these  folds  to  the  lateral  wall  of  the  abdomen  constitute  the 
lateral  ligaments  of  the  liver. 

The  rotation  of  the  stomach  to  assume  its  permanent  rela- 
tions alters  the  position  of  the  fold  that  connects  its  lesser 
curvature  with  the  liver,  bringing  this  fold  into  a  plane  par- 
allel, approximately,  with  the  ventral  wall  of  the  abdomen. 
This  fold  is  now  the  lesser  or  gastrohepatic  omentum. 

The  round  ligament  of  the  adult  represents  the  impervious 
vestige  of  the  umbilical  vein.  This  vessel,  entering  the  fetal 
body  at  the  umbilicus  and  passing  to  the  under  surface  of  the 
liver,  diverges  from  the  abdominal  wall  to  reach  that  organ 
and,  in  doing  so,  carries  with  it  the  parietal  peritoneum. 
The  fold  thus  formed  is  the  falciform  or  suspensory  ligament. 

13 


194  TEXT-BOOK  OF  EMBRYOLOGY. 

The  special  system  of  blood-vessels  belonging  to  the  liver 
is  described  in  the  chapter  on  the  Vascular  System,  p.  161. 

THE  DEVELOPMENT  OF  THE  PANCREAS. 

Just  as  the  liver  is  produced  by  an  evagination  from  the 
ventral  wall  of  the  future  duodenum,  so  is  the  pancreas 
originated  by  an  evagination  from  the  dorsal  wall  of  the 
same  region  of  the  gut-tube  (Fig.  86).  The  diverticulum 
grows  between  the  two  layers  of  the  dorsal  mesentery  (Figs. 
86,  88,  and  90),  where  it  encounters  embryonic  connective 
tissue,  the  cells  and  the  blood-vessels  of  which,  becoming 
associated  with  the  epithelial  entodermic  evagination,  give 
rise  to  all  the  vascular  and  connective-tissue  parts  of  the 
gland.  The  details  of  development  correspond  closely  with 
those  of  the  salivary  glands,  with  which  the  pancreas  is  prac- 
tically identical  in  structure.  The  stalk  of  the  diverticulum 
becomes  the  duct  of  the  mature  gland.  Although  the  orifice 
of  the  duct,  in  early  stages,  is  on  the  opposite  side  of  the 
bowel  from  that  of  the  common  bile-duct,  the  two  apertures 
are  made  to  approach  each  other  and  are  finally,  in  most 
cases,  merged  into  one  by  the  unequal  growth  of  the  lateral 
walls  of  the  intestine.  In  the  adult,  therefore,  the  duct  of 
the  pancreas  and  that  of  the  liver  and  gall-bladder  ojDen  into 
the  duodenum  by  a  common  orifice. 

Since  the  pancreas  develops  between  the  folds  of  the  dorsal 
mesentery,  it  has  a  complete  investment  of  peritoneum  in  the 
early  stages  of  fetal  life.  The  alterations  by  which  this  con- 
dition is  changed,  making  the  pancreas  a  retroperitoneal 
organ,  will  be  noted  in  the  account  of  the  peritoneum. 

THE  DEVELOPMENT  OF  THE  SPLEEN. 

Although  the  spleen  does  not  belong  to  the  digestive  sys- 
tem, it  may  conveniently  be  considered  here  because  of  its 
position  and  relations. 

This  organ  is  diiferentiated  from  the  mesodermic  tissue 
found  between  the  layers  of  the  mesogastrium  in  close 
proximity  to  the  developing  pancreas  (Fig.  90).  Prim- 
itively, therefore,  it  is  situated  behind  the  stomach.      The 


THE  EVOLUTION  OF  THE  PERITONEUM.  195 

first  step  in  its  development,  recognizable  at  about  the 
end  of  the  second  month,  is  the  accumulation  of  numerous 
lymphoid  cells  with  large  granular  nuclei.  The  mass  is  aug- 
mented by  the  addition  of  cells  immediately  beneath  the 
peritoneal  surfaces  of  the  mesogastrium,  which  cells  elongate 
until  they  are  spindle-shaped  and  then  become  aggregated 
into  fusiform  masses.  Blood-vessels  penetrate  the  fundament 
in  the  third  month  and  become  surrounded  by  cells  of  the 
same  spindle-shaped  type.  From  both  the  cells  surrounding 
the  blood-vessels  and  from  those  of  the  fusiform  aggrega- 
tions, processes  grow  out  and  unite  with  each  other,  and  from 
the  network  thus  formed  the  trabecular  framework  of  the 
organ  is  ultimately  evolved.  Accumulations  of  small  nucle- 
ated cells,  forming  dense  masses  along  the  arteries,  furnish 
the  chief  constituent  of  the  pulp.  The  delicate  intercellular 
substance  which  makes  up  the  remainder  of  the  pulp  is  filled 
with  blood-corpuscles.  The  Malpighian  corpuscles  appear 
before  the  end  of  the  fourth  month.  By  the  sixth  month, 
the  spleen  attains  its  characteristic  shape  and  the  fibrous  cap- 
sule is  clearly  indicated. 

The  spleen  undergoes  a  change  of  location  coincident  with 
the  rotation  of  the  stomach  and  the  alteration  of  the  meso- 
gastrium.  The  organ  being  from  the  first  embedded  within 
the  mesogastrium,  it  follows  that  peritoneal  fold  to  the  left 
side  of  the  abdominal  cavity.  Here  it  lies  close  to  the 
cardiac  end  of  the  stomach,  between  the  two  layers  of  the 
mesogastrium,  but  projecting  toward  the  left.  The  part  of 
the  mesogastrium  which  intervenes  between  the  spleen  and 
the  stomach  is  the  gastrosplenic  omentum ;  while  the  part  that 
passes  from  the  spleen  to  the  posterior  wall  of  the  abdomen, 
representing  the  parietal  attachment  of  the  mesogastrium, 
constitutes  the  phrenicosplenic  omentum. 

THE  EVOLUTION  OF  THE  PERITONEUM. 

The  arrangement  of  the  peritoneum  being  subordinate  to 
the  position  and  relations  of  the  viscera  contained  within  the 
abdomen,  the  development  of  this  complex  membrane  can  be 
properly  described  only  by  tracing  the  growth  of  the  digestive 


196 


TEXT-BOOK  OF  EMBRYOLOGY. 


system.  As  the  formation  of  the  early  gut- tube  by  the  in- 
folding of  the  splanchnopleure  has  been  pointed  out  (pp. 
169  and  171),  we  may  begin  at  once  with  the  period  when  the 
tract  has  the  form  of  a  straight  tube  connected  with  the 


Fig.  91.— Reconstruction  of  human  embryo  of  about  seventeen  days  (after  His) : 
ov,  optic  and  ot,  otic  vesicles ;  nc,  uotochord ;  hdg,  head-gut ;  g,  mid-gut ;  hg,  hind- 
gut;  vs,  vitelline  sac;  I,  liver;  v,  primitive  ventricle;  va,  da,  ventral  and  dorsal 
aortEe;  jv,  primitive  jugular  vein;  cv,  cardinal  vein;  dC,  duct  of  Cuvier;  uv,  ua, 
umbilical  vein  and  artery ;  al,  allantois  ;  its,  umbilical  cord ;  dm,  dorsal  mesentery ; 
vm,  ventral  mesentery  (modified  from  His). 

dor.sal  and  the  ventral  body-wall  respectively  by  the  dorsal 
and  the  ventral  mesentery  (Fig.  91).  Covering  the  tube  as  a 
constituent  part  of  its  wall,  is  the  splanchnic  or  visceral  layer 
of  the  mesoderm,  while  the  somatopleuric  or  parietal  layer 
of  the  latter  lines  the  wall  of  the  body.     Obviously  these 


THE  EVOLUTION  OF  THE  PERITONEUM. 


197 


two  lamellae  of  the  mesoderm  are  continuous  with  each  other 
through  the  medium  of  the  mesenteries  mentioned  above 
(Fig.  92,  A  and  B).  The  space  thus  enclosed  by  the  meso- 
dermic  strata  is  the  body-cavity  or  coelom  or  pleuroperitoneal 
cavity.  The  surface-cells  of  both  strata  flatten  and  assume 
the  character  of  mesothelial,  the  later  endothelial,  cells.     If, 


Fig.  92.— a,  B,  two  transverse  sections,  A  through  thoracic,  B  through  abdom- 
inal region ;  C,  sagittal  section  (Tourneux) :  1,  dorsal  mesentery ;  2,  ventral  mesen- 
tery ;  3,  mesocardium  posterius ;  4,  mesocardium  anterius ;  5,  lesser  omentum ;  6, 
suspensory  ligament  of  liver;  7,  esophagus;  8,  lungs;  9,  heart;  10,  pancreas;' 11, 
stomach ;  12,  liver ;  13,  spleen ;  14,  loop  of  intestine  with  vitelline  duet ;  15,  caecum ; 
16,  trachea. 

at  this  stage,  one  begins  at  any  point  to  trace  the  mesothelial 
lining  of  the  body-cavity,  that  lining  is  found  to  form  prac- 
tically one  continuous  sheet. 

This  simple  arrangement  of  the  primitive  peritoneum  is 
transformed  into  the  complicated  membrane  of  the  adult, 
primarily,  by  the  increase  in  length  and  consequent  tortuosity 
of  the  alimentary  tube ;  and,  secondarily,  by  the  fact  that 
certain  opposed  portions  of  the  serous  membrane,  which  have 
been  brought  into  contact  by  the  altered  relations  of  the 
bowel  and  the  stomach,  undergo  concrescence  or  fusion 
with  each  other.  Simultaneously  with  these  alterations,  the 
original  pleuroperitoneal  cavity  suffers  division  into  the  ab- 


198  TEXT-BOOK  OF  EMBRYOLOGY. 

dominal  or  peritoneal  cavity  and  the  thoracic  part  of  the 
body-cavity  by  the  development  of  the  diaphragm.  This  is 
described  on  p.  158. 

The  first  modification  of  the  original  arrangement  is  effected 
by  the  development  of  the  stomach  as  a  spindle-shaped  dila- 
tation of  the  gut-tube,  differentiating  the  tube  into  the 
stomach  and  the  intestine,  and  the  common  dorsal  mesentery 
into  the  mesogastrium  and  the  intestinal  mesentery.  The 
drawing  out  of  the  U-shaped  loop  of  the  intestine  from  the 
dorsal  body-wall,  which  is  the  preliminary  step  to  the  dis- 
tinction between  the  small  intestine  and  the  colon,  increases 
the  length  of  the  intestinal  mesentery  to  a  corresponding 
extent  (Fig.  92,  C).  As  heretofore  indicated,  the  lower 
limb  of  the  loop  presents  an  enlargement  which  is  the  begin- 
ning of  the  development  of  the  large  intestine. 

An  important  stage  in  the  evolution  of  the  peritoneum  is 
marked  by  the  rotation  of  the  stomach  and  by  the  migration 
of  the  proximal  part  of  the  large  intestine  to  a  new  location. 
The  change  of  position  on  the  part  of  the  colon  may  perhaps 
be  best  expressed  by  saying  that  the  U-loop  of  intestine 
rotates  upon  an  oblique  dorsoventral  axis,  whereby  the  lower 
limb  of  the  loop,  in  other  words,  the  termination  of  the  small 
bowel  and  the  beginning  of  the  colon,  is  carried  to  a  position 
above,  cephalad  to,  the  upper  limb  (Fig.  89,  A).  This  rota- 
tion brings  the  beginning  of  the  colon  into  the  right  hypo- 
chondriac region  of  the  abdomen,  from  which  point  the 
transverse  colon  passes  across  the  abdominal  cavity,  ventrad 
to  the  proximal  end  of  the  small  intestine  or  duodenum.  As 
a  consequence  of  the  altered  situation  of  the  transverse  part 
of  the  colon,  its  mesentery  shifts  its  area  of  attachment  by 
fusing  with  the  peritoneum  of  the  dorsal  wall  along  a  hori- 
zontal line  and  also  with  that  of  the  ventral  surface  of  the 
duodenum.  The  descending  colon  having  meanwhile  moved 
to  the  left,  its  mesentery  likewise  acquires  a  new  area  of 
attachment  by  concrescence  with  the  parietal  peritoneum  of 
the  dorsal  wall  of  the  abdomen  on  the  left  side.  During  the 
progress  of  these  alterations,  the  small  intestine  increases  in 
length,   and   its   mesentery   becomes   correspondingly   more 


THE  EVOLUTION  OF  THE  PERITONEUM. 


199 


voluminous  both  in  the  extent  of  its  intestinal  border  and 
in  length.  The  convolutions  of  the  small  intestine  now 
occupy  the  space  below  the  transverse  colon  and  its  mesen- 
tery. 

The  duodenum,  which  in  the  early  stage  shares  with  the 
gastro-intestinal  tube  in  the  possession  of  the  common  dorsal 
mesentery,  loses  its  mesenterial  connection  with  the  abdomi- 
nal wall  and  becomes  thereby  a  fixed  part  of  the  intestine. 
Mention  was  made  above  of  the  fusion  of  the  transverse 
mesocolon  with  the  peritoneum  of  the  ventral  surface  of  the 
duodenum.  At  about  the  same  time,  the  duodenal  mesentery 
(Fig.  89,  A)  fuses  with  the  parietal  peritoneum  of  the  poste- 
rior abdominal  wall,  the  result  being  that  the  lower  layer  of 


Fig.  93.—^,  B.  Two  successive  stages  of  the  development  of  the  mesenteries 
(schematic  representation  showing  sagittal  axial  section  of  truni:,  after  Gegenbaur 
and  Hertwig):  1,  stomach ;  2,  duodenum;  3,  transverse  colon;  4,  small  intestine; 
5,  pancreas ;  6,  liVer ;  7,  lesser  omentum ;  8, 10,  different  stages  of  great  omentum ; 
9,  transverse  mesocolon ;  11,  mesentery ;  12,  suspensory  ligament  of  liver ;  13,  cav- 
ity of  omental  bursa  or  lesser  peritoneal  sac ;  14,  diaphragm. 

the  transverse  mesocolon,  as  it  passes  downward,  is  now  con- 
tinuous with  the  parietal  peritoneum,  there  being  no  longer 
any  serous  membrane  between  the  transverse  part  of  the 
duodenum  and  the  abdominal  wall  (Fig.  93,  B).  This  part 
of  the  duodenum  therefore  becomes  retroperitoneal,  there 
being  an  investment  of  serous  membrane  only  on  its  anterior 
or  ventral  surface. 


200  TEXT-BOOK  OF  EMBRYOLOGY. 

The  second  modifying  factor  iu  the  complication  of  the 
peritoneum,  the  rotation  of  the  stomach,  initiates  alterations 
in  its  mesogastrium.  The  latter  membrane,  it  will  be  re- 
membered, is  a  vertical  median  fold  of  peritoneum  con- 
tinuous with  the  mesentery  of  the  duodenum  (Fig.  92,  C). 
As  the  stomach  moves  about  its  two  axes  of  rotation,  the 
mesogastrium  begins  to  sag  toward  the  left  (Fig.  89),  so  that 
now  it  constitutes  a  pouch  or  fossa,  the  omental  bursa,  situ- 

Orifice  of  ome77tal  bursa.      Spinal  cord.     Aorta. 

Right  lobe  of  liver.  ■/ 1  ^^^^^^^^^^m%. |m\\    l^^ft  ^obe  of  liver. 

Lesser  oinentum.         Stomach. 
Fig.  94. — Schematic  cross-section  tlirough  body  of  mammalian  embryo  In  region 
of  stomach,  to  show  development  of  omental  bursa  (after  Toldt). 

ated  between  the  stomach  and  the  dorsal  body-wall,  the 
opening  of  which  looks  toward  the  right  side  of  the  body 
(Figs.  93,  A  and  94).  With  the  rapidly  increasing  re- 
dundancy of  the  mesogastrium,  the  omental  bursa  becomes 
more  and  more  capacious.  In  correspondence  with  the  pro- 
gressive rotation  of  the  stomach,  what  was  at  first  the  left 
surface  of  the  mesogastrium  comes  into  contact  with  the 
peritoneum  of  the  dorsal  abdominal  wall  and  fuses  with  it, 
thus  changing  its  area  of  parietal  attachment  from  a  median 
vertical  line  to  a  transverse  one.  This  change  is  completed 
by  the  time  the  stomach  has  attained  its  normal  adult  posi- 
tion. The  omental  bursa  now  has  the  position  and  relations 
shown  in  Fig.  93,  A,  8.  A  still  further  increase  in  the  size 
of  the  bursa  results  in  its  protrusion  downward  in  front  of, 
ventrad  to,  the  transverse  colon  and  the  small  intestine.  Kef- 
erence  to  Fig.  93,  B  will  show  that  the  dependent  part  of 
the  bursa  very  nearly  corresponds  with  the  fully  formed 
great  omentum.  It  will  be  seen,  however,  that  the  deeper 
layer  of  the  bursa,  the  layer  nearer  the  intestines,  may  be 
traced  above  the  transverse  colon  and  its  mesentery  to  the 


THE  EVOLUTION  OF  THE  PERITONEUM.  201 

dorsal  wall  of  the  abdomen,  where  its  two  lamellse  separate 
to  enclose  the  pancreas,  one  lamina  passing  over  the  ventral 
surface  of  the  pancreas  to  become  continuous  with  the  parietal 
peritoneum,  while  the  other  layer  passes  between  the  pancreas 
and  the  abdominal  wall.  The  latter  layer  is  in  continuity 
here  with  the  parietal  peritoneum,  which  almost  immediately 
leaves  the  abdominal  wall  to  form  the  upper  layer  of  the 
transverse  mesocolon. 

The  further  alterations  necessary  for  the  attainment  of 
the  completed  condition  consist  in  the  concrescence  of 
certain  opposed  peritoneal  surfaces.  As  a  conspicuous  ex- 
ample of  such  concrescence,  the  deeper  lamella  of  the  layer 
of  the  omental  bursa  just  described  fuses  with  the  ventral 
peritoneal  surface  of  the  transverse  colon  and  with  the 
upper  layer  of  the  transverse  mesocolon  (Fig.  93,  A),  after 
which  event  this  deeper  lamella  is  practically  continuous 
with  the  lower  layer  of  the  mesocolon,  while  the  superficial 
lamella  is  in  continuity  with  the  upper  layer  of  the  meso- 
colon (Fig.  93,  B).  Thus  the  transverse  colon  appears  as  if 
enclosed  between  the  two  lamellae  of  the  deeper  layer  of  the 
great  omentum,  while  its  mesocolon  is  constituted  by  a  part 
of  the  same  structure.  In  other  words,  the  adult  transverse 
mesocolon  includes  not  only  the  primitive  membrane  of  that 
name  but  also  a  part  of  the  early  mesogastrium.  Similarly, 
the  opposed  surfaces  of  peritoneum  between  the  pancreas  and 
the  dorsal  abdominal  wall  undergo  fusion  (Fig.  93),  the  effect 
of  which,  after  the  concrescence  of  the  mesocolon  with  the 
deeper  layer  of  the  omental  bursa,  is  to  make  the  lower  layer 
of  the  mesocolon  continuous,  over  the  transverse  part  of  the 
duodenum,  with  the  parietal  peritoneum. 

The  great  omentum  of  descriptive  anatomy,  resulting  from 
the  downwardly  projecting  process  of  the  omental  bursa, 
consists  originally  of  two  layers  of  membrane,  each  one 
having  two  serous  surfaces.  At  the  time  of  birth  these 
two  layers  are  still  separate — the  permanent  condition  in 
some  mammals — but  during  the  first  year  or  two  after  birth 
they  become  adherent,  the  great  omentum  thus  coming  to 
comprise  but  a  single  layer. 


202  TEXT-BOOK  OF  EMBRYOLOGY. 

It  remains  to  note  the  metamorphosis  of  the  ventral  mesen- 
tery, which,  prior  to  the  rotation  of  the  stomach,  is  a  vertical 
median  fold  connecting  the  lesser  curvature  of  that  viscus 
with  the  ventral  abdominal  wall.  Since  the  evagination  of 
the  gut-tube  that  gives  rise  to  the  liver  grows  between  the 
layers  of  the  ventral  mesentery  to  reach  the  septum  trans- 
versum,  the  liver  is  not  only  enclosed  by  the  mesentery,  but 
is  connected  by  it  with  the  stomach  and  with  the  ventral 
wall  of  the  abdomen  and  also  with  the  primitive  diaphragm 
(Fig.  92).  By  the  rotation  of  the  stomach,  the  vertical 
median  fold  which  connects  that  organ  with  the  liver  becomes 
so  altered  in  position  as  to  lie  in  a  plane  approximately  par- 
allel with  the  ventral  surface  of  the  body.  This  fold  is  now 
the  gastrohepatic  or  lesser  omentum.  As  reference  to  Fig. 
93  will  show,  it  is  the  anterior  boundary,  above  the  position 
of  the  stomach,  of  the  sac  described  above  as  the  omental 
bursa. 

That  part  of  the  ventral  mesentery  that  connects  the  liver 
with  the  abdominal  wall  and  with  the  diaphragm,  while 
originally  occupying  the  median  plane,  is  modified  by  the 
relation  of  the  developing  liver  to  the  primitive  diaphragm. 
These  organs  are  intimately  united  with  each  other  (p.  159) 
in  the  early  stage  of  their  growth,  but  with  their  completion 
a  reparation  takes  place.  Upon  the  two  separated  surfaces, 
except  in  a  region  near  the  dorsal  wall,  the  cells  assume  the 
endothelial  type,  the  opposed  surfaces  thus  acquiring  the 
characters  of  serous  membrane.  The  peritoneum  on  the 
under  surface  of  the  diaphragm  is  continuous  with  that  on 
the  upper  surface  of  the  liver,  both  in  front  of  and  behind 
the  non-peritoneal  area  of  contact.  Therefore,  in  the  com- 
pleted condition  of  the  liver  and  the  diaphragm,  these  two 
structures  are  connected  by  two  layers  of  peritoneum  sepa- 
rated from  each  other  by  a  region  containing  only  areolar 
tissue.  These  layers  constitute  the  coronary  ligament  of  the 
liver.  If  now  Fig.  93  is  inspected,  it  will  be  seen  that  the 
posterior  layer  of  the  lesser  omentum,  and  the  upper  layer 
of  the  transverse  mesocolon,  together  with  that  part  of  the 
peritoneum  with  which  they  are  in  direct  continuity,  enclose 


THE  EVOLUTION  OF  THE  PERITONEUM.  203 

a  sac  which  is  the  so-called  lesser  bag  of  the  peritoneum  or 
the  lesser  peritoneal  cavity.  All  other  parts  of  the  perito- 
neum taken  together  constitute  the  greater  peritoneal  cavity. 
The  communication  between  the  two,  the  foramen  of  Winslow, 
situated  behind  the  free  right  border  of  the  lesser  omentum, 
is  the  constricted  orifice  of  the  early  omental  bursa. 

The  position  of  the  kidneys  and  the  ureters  as  retroperi- 
toneal structures  and  the  relations  of  the  bladder  and  of  the 
uterus  to  the  peritoneum,  encroaching  as  they  do  upon  the 
parietal  layer  of  this  membrane,  and  being,  therefore,  in- 
vested by  it  to  a  greater  or  less  extent,  are  easily  accounted 
for  when  it  is  recalled  that  all  these  organs  develop  from  the 
somatic  or  outer  layer  of  the  mesoderm. 

The  peritoneum  does  not  acquire  all  the  characteristic 
features  of  a  serous  membrane  until  about  the  third  month. 
The  histological  alterations  begin  in  the  fourth  week,  from 
which  time  until  the  sixth  week  the  superficial  cells,  the 
mesothelium,  pass  through  various  phases  of  transition  to 
reach  the  condition  of  somewhat  flattened  elements.  By  the 
eighth  week  they  have  acquired  the  form  of  true  endothe- 
lium. It  is  not,  however,  until  the  third  month  that  the 
subjacent  tissue  has  attained  to  the  condition  of  a  fully- 
formed  basement  membrane. 


CHAPTER    XII. 

THE    DEVELOPMENT    OF    THE    RESPIRATORY 
SYSTEM. 

Although  the  nasal  chambers  and  the  pharyngeal  cavity 
contribute  to  the  formation  of  the  respiratory  system,  these 


Middle  lobe 

of  thyroid  gland. 

Thymus  gland. 

Lateral  lobe 

of  thyroid  gland. 

Trachea. 
Lung. 


Right  lobe  of  liver. 


Vitelline  duct. 


Pharyngeal 
pouches. 


Stomach. 

Pancreas. 

Left  lobe  of  liver. 

Small  intestine. 


Large  intestine. 


Fig.  95.— Scheme  of  the  alimentary  canal  and  its  accessory  organs  (Bonnet). 

parts  will  not  be  considered  here,  since  they  are  described 
elsewhere. 

204 


DEVELOPMENT  OF  THE  EESPIRATOBY  SYSTEM.    205 


Anatomically  and  according  to  their  mode  of  development, 
the  lungs  might  be  looked  upon  as  a  pair  of  glands  having  a 
common  duct,  the  trachea,  which  latter,  through  the  medium 
of  its  dilated  proximal  extrem- 
ity, the  larynx,  opens  into  the 
pharyngeal  cavity.  In  point 
of  fact,  these  organs  are  devel- 
oped as  an  outgrowth  from  the 
entodermal  alimentary  canal  in 
a  manner  similar  to  the  devel- 
opment of  the  liver  and  the 
pancreas. 

The  first  step  in  the  devel- 
opment of  the  lungs  is  the  out- 
pouching of  the  ventral  wall 
of  the  esophagus  throughout 
its  entire  length.  _  The  lon- 
gitudinal median  groove  thus 
formed  is  the  pulmonary  groove. 
It  makes  its  appearance  when 

the  embryo  has  a  length  of  3.2  mm.  (0.128  inch)  or  prob- 
ably early  in  the  third  week.  The  groove  is  more  pro- 
nounced at  its  lower  or  gastric  extremity.  As  the  groove 
deepens,  its  edges  approach  and  finally  meet  and  fuse 
with  each  other.  In  this  manner  the  groove  is  converted 
into  a  tube,  which  gradually  separates  from  the  esophagus, 
the  separation  beginning  at  the  end  toward  the  stomach  and 
progressing  toward  the  pharynx.  The  separation,  however, 
is  not  complete,  stopping  short  of  the  upper  end  of  the 
groove,  so  that  the  tube  retains  communication  with  the 
pharyngeal  end  of  the  esophagus.  Even  before  the  con- 
stricting off  of  this  tube  or  pulmonary  diverticulum  is  com- 
pleted, its  free  end  bifurcates.  The  pulmonary  anlage  con- 
sists, then,  at  this  stage,  of  two  short  wide  pouches  connected 
by  a  common  pedicle  with  the  primitive  pharynx  (Figs.  95 
and  96),  and  this  condition  is  present  in  the  fourth  week. 

Very  soon  after  the  end  of  the  first  month  each  of  the 
pouches  undergoes  division,  the  right  one  into  three  branches, 


Fig.  96. — Transverse  section  to  show 
outgrowth  of  pulmonary  anlage  from 
gut-tube  (after  Tourneuxjrr^,  dorsal 
mesentery;  2,  ventral  mesentery  in- 
cluding 3,  mesocardium  posterius  ;  4, 
mesocardium  anterius ;  7,  esophagus; 

8,  diverticulum  which   becomes   the 
lungs,  the  trachea,  and  the  larynx; 

9,  heart. 


206 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  left  one  into  two,  while  at  the  same  time  they  increase 
in  size  (Fig.  97).     The  further  steps  toward  the  attainment 


Fig.  97.— View  of  a  reconstruction  of  the  fundament  of  the  lungs  of  a  human 
embryo  (Pr.  of  His)  10  mm.  long,  neck  measurement  (after  His) :  Ir,  trachea ;  br,  right 
bronchus;  sp,  esophagus;  &/,  connective-tissue  envelope  and  serous  membrane 
(pleura)  into  which  the  epithelial  fundament  of  the  lung  grows ;  0,  M,  U,  funda- 
ments of  the  upper,  middle,  and  lower  lobes  of  the  right  lung;  Oi,  Fi,  fundaments 
of  the  upper  and  lower  lobes  of  the  left  lung. 

of  the  completed  condition  consist  largely  in  the  continued 
repetition  of  this  process  of  dichotomous  division  (Fig.  98), 


Fig.  98.— View  of  reconstruction  of  the  fundament  of  the  lungs  of  a  human 
embryo  (N.  of  His)  older  than  that  of  Fig.  97  (after  His,  magnified  50  diameters) : 
Ap,  arteria  pulmonalis  ;  Ir,  trachea ;  sp,  esophagus ;  lb,  pulmonary  vesicle  in  process 
of  division ;  0,  upper  lobe  of  the  right  lung  with  an  eparterial  bronchus  leading 
to  it ;  M,  V,  middle  and  lower  lobes  of  the  right  lung ;  0',  upper  lobe  of  the  left  lung 
with  hyparterial  bronchus  leading  to  it ;  [/',  lower  lobe  of  the  left  lung. 

which  latter  goes  on  until  the  sixth  month.  The  original 
evagination,  consisting  of  entodermal  epithelium,  gives  rise 
only  to  the  epithelial  parts  of  the  lungs  and  air-passages. 
All  the  other  constituents,  the  connective  tissue,  the  mus- 
cular, vascular,  and  cartilaginous  elements,  are  products  of 
the  mesodermic  tissue  into  which  the  diverticulum  grows. 
Upon  their  fir.st  appearance  the  "tubes"  are  always  solid 


THE  THYROID  AND   THE  THYMUS  BODIES.        207 

epithelial  cylinders,  the  lumina  being  acquired  later.  At 
first,  the  lining  entodermal  cells  of  the  primitive  tubes  are 
tall  and  cylindrical,  the  tubes  themselves  having  a  relatively 
small  lumen.  In  the  fourth  month  the  cells  acquire  cilia. 
From  the  anatomical  standpoint,  the  lungs  now  present  the 
characters  of  compound  saccular  glands. 

From  the  sixth  month  to  the  end  of  gestation  occur  the 
changes  which  give  to  the  organs  their  essential  character- 
istics. Upon  the  dilated  extremity  of  each  terminal  tube 
numerous  little  evaginations  develop.  These  are  the  air-sacs, 
or  pulmonary  alveoli,  the  terminal  tubes  from  which  they  are 
evaginated  being  the  alveolar  passages  and  the  infundibula. 
Their  walls  remain  very  thin  and  their  lining  epithelium 
flattens  to  such  a  degree  as  to  closely  resemble  endothelium. 
The  trachea  is  simply  the  elongated  stalk  of  the  pulmonary 
diverticulum. 

The  larynx  is  the  dilated  proximal  extremity  of  the  pedicle, 
specially  developed  to  serve  as  an  organ  of  phonation.  One 
of  the  earliest  changes  is  the  appearance  of  two  ridges  at 
the  junction  of  the  primitive  trachea  with  the  esophagus. 
These  are  close  together  in  front,  ventrally,  but  separated 
behind.  They  are  the  first  indication  of  the  vocal  cords. 
The  arytenoid  cartilages  are  indicated  in  the  sixth  week  and 
the  other  cartilages  soon  after.  These  are  not  actually  car- 
tilaginous, however,  until  the  eighth  or  ninth  week. 

The  development  of  the  pleurae  has  been  described  in  con- 
nection with  that  of  the  pericardium  and  of  the  diaphragm 
(p.  158). 

THE  THYROID  AND  THE  THYMUS  BODIES. 

These  organs  may  be  considered  in  this  connection,  as  a 
matter  of  convenience  and  because  of  their  embryologieal 
relationship  to  the  respiratory  system,  being  developed,  like 
the  latter,  from  the  epithelium  of  the  gut-tract. 

The  thyroid  body,  an  organ  common  to  all  vertebrates, 
genetically  consists  of  two  parts,  a  median  lobe  and  two  lateral 
lobes. 

The  median  portion  originates  from  an  evagination  of  the  ven- 


208 


TEXT-BOOK  OF  EMBRYOLOGY. 


tral  wall  of  the  pharynx,  in  the  median  line,  posterior,  caudad, 
to  the  tuberculum  impar,  and  between  the  ventral  extremities 
of  the  first  and  second  visceral  arches.  This  median  divert- 
iculum is  present  in  the  human  embryo  of  5  mm.  It  soon 
pouches  out  on  either  side,  assuming  thereby  the  form  of  an 
epithelial  vesicle  connected  by  the  constricted  pedicle  of  the 
diverticulum  with  the  ventral  wall  of  the  pharynx  (Fig.  99, 3). 


Fig.  99.— Diagrammatic  representation  of  pharynx  of  human  embryo  seen  from 
in  front  (after  Tourneux) :  I,  II,  first  and  second  pharyngeal  pouches ;  1,  tuberculum 
impar ;  2,  course  of  thyroglossal  duct  leading  from  3,  median  lobe  of  thyroid  gland ; 
4,  laryngotracheal  tube ;  5,  esophagus ;  6,  thymus  ;  7,  accessory  thymus ;  S,  lateral 
lobe  of  thyroid ;  9,  accessory  thyroid. 

From  the  situation  of  the  original  point  of  evagination  be- 
hind the  tuberculum  impar  and  ventromesial  to  the  two 
halves  of  the  posterior  segment  of  the  tongue,  the  orifice  of 
the  pedicle  corresponds  to  the  line  of  junction  of  the  three 
parts  of  the  tongue.  As  a  consequence,  when  these  parts 
unite,  the  pedicle  or  duct  is  prolonged  upward  and  comes  to 
open  upon  the  surface  of  the  tongue.  The  canal  is  known 
as  the  thyroglossal  duct  or  canal  of  His.  In  the  fifth  week 
it  begins  to  atrophy,  and  usually  by  the  eighth  week  has  be- 
come obliterated.  Occasionally  it  persists  throughout  life. 
The  foramen  csecum  on  the  dorsum  of  the  tongue  is  the 
vestige  of  the  orifice  of  the  duct.  Other  vestiges  of  the 
thyroglossal  duct  are  sometimes  present.  For  example,  the 
lower  part  of  the  duct  may  persist  as  a  short  tube,  the 
thyroid  duct,  leading  upward  from  the  median  lobe  to  the 


THE  THYROID  AND   THE  THYMUS  BODIES.        209 

hyoid  bone;  and  again,  according  to  His,  isolated  persistent 
segments  of  the  duct  constitute  the  little  vesicles  in  the 
neighborhood  of  the  hyoid  bone  which  are  known  respect- 
ively as  the  accessory  thyroid,  and  the  suprahyoid  and  pre- 
hyoid  glands. 

The  lateral  lobes  of  the  thyroid  body  begin  their  develop- 
ment somewhat  later  than  does  the  median  lobe.  In  the 
embryo  of  10  mm.,  the  fourth  inner  visceral  furrow  or  throat- 
pocket  of  each  side  pouches  out  to  form  a  vesicle  (Fig.  99, 
8).  As  the  vesicle  grows,  its  pedicle  becomes  attenuated  and 
finally  disappears.  After  their  isolation  from  the  throat- 
pockets,  the  vesicles  give  out  small  bud-like  processes  after 
the  usual  manner  of  the  development  of  glands  and  gradu- 
ally approach  the  median  lobe  (Fig.  100,  B).    The  three  parts 


Fig.  100.— Semi-diagrammatic  illustrations  to  show  the  ultimate  position  of  the 
thymus,  thyroid  gland  and  accessory  thyroid  gland  on  the  neck  of  the  chick  {A) 
and  the  calf  (B),  after  de  Meuron ;  sd,  thyroid  gland ;  nsd,  accessory  thyroid  gland ; 
th,  thymus ;  th',  accessory  thymus ;  Ir,  trachea ;  h,  heart ;  vj,  vena  jugularis ;  ca, 
carotid  vein. 

unite  probably  in  the  seventh  week.  In  the  vertebrates 
below  mammals  the  lateral  parts  of  the  thyroid  do  not  unite 
with  the  median  segment,  and  in  certain  animals  they  remain 
widely  separated  from  it  as  the  suprapericardial  bodies. 

After  the  union  of  the  three  portions  of  the  gland,  the  latter 
consists  of  a  network  of  cords  of  cells,  the  meshes  of  which 
reticulum  are  occupied  by  embryonal  connective  tissue.  Sub- 
sequently the  cords  of  cells  become  hollowed  out  and  exhibit 

14 


210  TEXT-BOOK  OF  EMBRYOLOGY. 

alternating  enlargements  and  constrictions.  By  the  increase 
of  the  constrictions  the  continuity  of  the  cell-cords  is  inter- 
rupted at  short  intervals,  and  so  the  network  is  converted 
into  numerous  closed  follicles  lined  with  epithelium,  the  forma- 
tion of  follicles  beginning  in  the  eighth  week.  The  follicles 
later  undergo  considerable  increase  in  size  on  account  of  the 
secretion  by  their  epithelial  cells  of  a  peculiar  colloid  material, 
characteristic  of  the  thyroid  body.  The  embryonal  connective 
tissue,  made  up  necessarily  of  mesodermic  elements,  furnishes 
the  connective-tissue  framework  and  the  blood-vessels  of  the 
organ,  while  the  epithelium  originates  in  the  manner  indicated 
from  the  entoderm  of  the  gut-tract. 

The  Thymus. — What  remains  of  the  thymus  after  the 
second  year  of  life  is  made  up  chiefly  of  lymphoid  and  con- 
nective tissue,  embedded  in  which  are  characteristic  little 
epithelial  bodies,  the  corpuscles  of  Hassall. 

The  epithelial  parts  of  the  thymus,  in  all  vertebrate  ani- 
mals, are  derived  from  the  entodermal  lining  of  the  pharyn- 
geal region  of  the  gut-tract.  In  the  lower  groups,  such  as 
reptiles,  amphibians,  and  bony  fishes,  the  epithelium  of  all 
the  inner  visceral  clefts  or  throat-pouches  shares  in  the  de- 
velopment ;  while  in  birds,  only  two  or  three  clefts  take 
part.  In  mammals,  however,  including  man,  the  thymus 
body  is  derived  probably  from  but  one  throat-pocket,  the 
third. 

The  entodermal  epithelium  of  the  third  inner  pouch  be- 
comes evaginated  (Fig.  99)  to  form  an  epithelial  sac  whose 
connection  with  the  pharyngeal  cavity  is  subsequently  lost. 
The  isolated  and  elongated  sac  soon  gives  out  small  lateral 
buds  or  processes  at  the  distal  extremity.  While  the  original 
sac  has  from  the  first  a  cavity,  the  bud-like  branches  are 
solid  masses  of  epithelium.  The  branching  continues  and 
affects  not  only  the  lower  or  distal  extremity  of  the  thymus 
sac  but  also  the  proximal  end,  the  structure  now  resembling 
an  acinous  gland  (Fig.  101).  While  this  growth  is  taking 
place,  the  epithelial  mass  is  being  invaded  by  lymphoid  cells 
and  young  connective  tissue  with  develo])ing  blood-vessels. 
The  encroachment  by  these  elements  continues  to  such  an 


THE  THYROID  AND   THE  THYMUS  BODIES.        211 


extent  that  lymphoid  tissue  becomes  the  predominant  con- 
stituent of  the  thymus,  the  epithelial  parts  suiFering  reduc- 
tion, relatively,  and  becoming  finally  broken  up  into  isolated 
masses  which  are  the  corpuscles  of 
Hassall  of  the  mature  gland.  The 
breaking  down  of  the  epithelial  cords 
is  probably  responsible  also  for  the 
irregular  cavities  of  the  thymus. 
Not  until  after  birth  do  the  glands 
of  the  two  sides  of  the  body  unite  to 
form  a  single  unpaired  structure,  and 
the  development  of  the  thymus  is 
not  completed  until  the  end  of  the 
second  year  of  life.  Having  attained 
its  full  development,  the  organ  begins 
to  retrograde,  and  at  the  time  of  pu- 
berty has  almost  disappeared.  Al- 
though sometimes  persistent  through- 
out life,  it  usually  suffers  complete 
obliteration,  or  at  most,  is  repre- 
sented by  an  insignificant  vestige. 
While  the  epithelial  parts  of  the  thy- 
mus body,  represented  in  the  fully 
developed  organ  by  the  corpuscles 
of  Hassall,  are  derived  from  the  ento- 
dermal  epithelium  of  the  third  inner 
visceral  furrow,  all  other  parts,  the 
lymphoid  tissue,  connective  tissue,  and  blood-vessels,  are  pro-^ 
ducts  of  the  surrounding  mesoderm. 


Fig.  101.— Thymus  of  an 
embryo  rabbit  of  sixteen 
days  (after  KoUiker),  magni- 
fied :  a,  canal  of  the  thymus ; 
h,  upper,  c,  lower  end  of  the- 
organ. 


CHAPTER    XIII. 

THE  DEVELOPMENT  OF  THE  QENITO=URINARY 
SYSTEM. 

Owing  to  the  intimate  anatomical  and  functional  associa- 
tion of  the  generative  organs  with  the  urinary  apparatus,  it 
is  necessary  to  discuss  the  development  of  these  two  systems 
together. 

THE  DEVELOPMENT  OF  THE  KIDNEY  AND  URETER. 

The  origin  of  the  kidney  and  ureter  of  the  higher  verte- 
brates is  associated  with  the  development  of  certain  fetal 
structures,  the  pronephros  and  the  mesonephros,  which  represent 
respectively  the  kidney  of  larval  amphibians  and  the  perma- 
nent kidney  of  fishes.  In  man  and  other  allied  types,  the 
former  structure  is  of  little  or  no  importance  functionally, 
while  the  latter  functionates  during  a  part  of  fetal  life  as  the 
organ  of  urinary  excretion,  prior  to  the  development  of  the 
permanent  kidney. 

The  pronephros  or  head-kidney  constitutes  the  most  primi- 
tive vertehrate  type  of  a  mechanism  for  the  excretion  of  urine. 
This  structure  has  its  origin  in  the  mesothelium  of  the  body- 
cavity,  and  in  the  following  manner :  When  the  paraxial 
mesoderm,  which  subsequently  divides  into  the  somites,  is 
about  to  separate  from  the  parietal  plate  of  mesoderm,  the 
two  parts  are  connected  for  a  time  by  an  intervening  thin 
band  of  tissue,  the  middle  plate  (Fig.  102,  mp).  On  the 
outer  side  of  this  middle  plate,  the  mesothelial  cells  of  the 
parietal  layer  of  the  mesoderm  become  invaginated  in  a  line 
parallel  with  the  axis  of  the  body  to  form  a  cord  of  cells 
which  at  several  points  retains  its  connection  with  the  surface 
cells.  Further  changes  bring  about  the  hollowing  out  of  the 
cell-cords  so  that  there  results  a  long  tube,  the  pronephric  or 

212 


DEVELOPMENT  OF  THE  KIDNEY  AND    URETER.     213 

segmental  duct,  which  has  several  short  transverse  tubules 
opening  into  it  and  communicating  by  their  opposite  ex- 
tremities with  the  body-cavity  (Fig.  103).  The  mesothelium 
in  immediate  proximity  to  the  free  end  of  each  short  tube 
is  invaginated  by  a  tuft  of  capillary  blood-vessels  from  a 
branch  of  the  adjacent  aorta  to  constitute  a  glomerulus  (Fig. 
103,  bb).  The  pronephric  duct  passes  tailward  and  opens 
into  the  cloaca,  a  receptacle  which  receives,  in  common,  the 
terminal  orifice  of  the  primitive  bladder  and  that  of  the 
primitive  intestine.      It  is  apparent,  therefore,  that  the  pro- 


Axial  zone.         ,  Neural  canal. 


Somite. 


Lateral  zone. 


Cavity  ■within  somite. 


Lateral  plates  for 
body-walls. 


Lateral  plates  for 
gut-tract. 


Parietal  mesoderm. 


_   fleuroperitoneal 
cavity. 


Vitelline  vein. 
Fig.  102.— Tranverse  section  of  a  seventeen-and-a-half-day  sheep  embrj'o  (Bonnet)* 


nephros  or  head-kidney  is  anatomically  adapted  to  the  func- 
tion of  removing  certain  substances  from  the  blood  by  virtue 
of  the  action  of  the  cells  surrounding  the  glomeruli  or  tufts 
of  capillary  blood-vessels,  and  that  these  substances  may  be 
conveyed  away  through  the  duct  into  the  cloaca  and  thence 
evacuated  from  the  body.  This  organ  is  functionally  active, 
however,  only  in  certain  lower  classes  of  vertebrates,  as  in 
the  Amphibia  during  the  larval  stage  and  in  bony  fishes.  In 
mammals  it  is  exceedingly  rudimentary  and  very  soon  gives 
place  to  a  more  important  organ,  the  mesonephros. 

The  Mesonephros  or  Wolffian  Body. — As  in  the  case  of  the 
pronephros,  the  origin  of  the  mesonephros  is  to  be  found  in 
the  parietal  mesothelium  of  the  body-cavity  or  coelom.  Ref- 
erence has  been  made,  in  treating  of  the  primitive  segments. 


214 


TEXT-BOOK  OF  EMBRYOLOGY. 


page  69,  to  the  middle  plate  (Fig.  102)  as  a  tract  of 
mesodermic  tissue  connecting  the  paraxial  tract  with  the 
parietal  plate.  When  the  paraxial  mesoderm  segments  to 
form  the  somites,  the  middle  plate  likewise  undergoes  seg- 
mentation, each  segment  being  designated  a  nephrotome. 
Each  nephrotome,  in  the  lower  vertebrates,  contains  a  cavity 


Fig.  103. — Diagram  of  pro- 
nephros (P)  and  pronephric  duct 
(Pd):  Al,  allantois;  G,  gut;  CI, 
cloaca;  bh,  glomeruli. 


Fig.  104.— Diagram  of  Wolffian 
body  and  duct :  Al,  allantois  ;  G 
gut ;  CI,  cloaca ;  K.  kidney  evagi- 
nation. 


which  communicates  with  the  general  body-cavity  and  which 
is  therefore,  in  effect,  an  evagination  of  the  mesothelium  of 
this  space.  In  mammals,  however,  as  well  as  in  reptiles  and 
birds,  the  nephrotome  is  a  solid  cord  of  cells.  By  the  hol- 
lowing out  of  these  cell-cords  or  nephrotomes,  a  series  of 
transversely  directed  tubules  is  formed,  each  nephrotome,  in 
fact,  becoming  converted  into  a  short  canal.  These  tubes 
acquire  connection  by  their  deeper  ends  with  the  previously 
formed  pronephric  duct  (Fig.  103),  which  is  known  here- 
after, therefore,  as  the  mesonephric  or  Wolffian  duct  (Fig.  104). 
The  latter  duct  and  the  short  transverse  tubules  which  open 
into  it  constitute  the  Wolffian  body  or  mesonephros  (Figs.  105 
and  106,  1).  Hence,  in  its  fully  developed  condition,  which 
is  attained  by  the  seventh  week  of   fetal   life   in   man,  the 


DEVELOPMENT  OF  THE  KTDNEY  AND    URETER.     215 


Wd 


Fig.  105.— Transverse  section  of  seventeen-day  sheep  embrj'o  (Bonnet) :  am, 
amnion ;  as,  amniotic  sac ;  n,  neural  canal ;  s,  somite  dififerentiated  into  muscle- 
plate;  Wd,  Wolffian  duct;  Wh,  Wolffian  body;  pm,  parietal  mesoderm;  vm,  vis- 
ceral mesoderm;  «,  a,  fusing  primitive  aortse;  i,  intestine. 

Wolffian  body  consists  of  a  tube  or  duct  lying  behind  the 
parietal  layer  of  the  mesoderm,  parallel  with,  and  lateral  to, 


Fig.  106.— Disposition  of  the  genito-urinary  organs  in  the  embryo  of  the  hog— 
5.5  cm  (2.2  in.)  long  (Tourneux) :  1,  Wolffian  body  ;  2,  ovary  ;  3,  inguinal  ligament ; 
4,  diaphragmatic  ligament;  5,  stomach  ;  6,  intestine  ;  7,  bladder;  8,  umbilical  artery. 

the  primitive  vertebral  column,  and  opening  at  the  caudal  end 
of  the  embryo  into  the  cloaca  ;  and  of  a  series  of  transverse 


216  TEXT-BOOK  OF  EMBRYOLOGY. 

Wolffian  tubules  opening  into  the  duct  and  abutting  by  their 
opposite  ends  upon  the  body-cavity.  At  the  head-end  of  the 
Wolffian  duct,  the  now  atrophic  pronephric  tubes  are  still  in 
connection  with  it. 

It  is  usual  for  writers  upon  embryology  to  compare  the 
general  form  of  the  Wolffian  body  to  a  comb,  the  short 
tubules  representing  the  teeth,  and  the  duct  corresponding  to 
the  back  of  the  comb. 

As  a  further  step  in  the  development  of  an  organ  adapted 
to  the  function  of  the  secretion  of  urine,  each  Wolffian  tubule 
becomes  somewhat  saccular  midway  between  its  two  extremi- 
ties, and  this  dilated  part  of  the  tubule  is  invaginated  by  the 
capillary  branches  of  an  artery  from  the  aorta.  In  this 
manner  the  cells  that  line  the  tubules  are  brought  into  rela- 
tion with  the  blood  of  the  fetus  and  acquire  at  the  same  time 
the  characters  of  secreting  epithelium.  Such  an  invaginating 
tuft  of  capillaries,  known  as  a  glomerulus,  with  its  enveloping 
capsule  of  Bo'\J|bn,  which  latter  is  the  invaginated  saccular 
part  of  the  tulKTO,  constitutes  a  primitive  Malpighian  cor- 
puscle, a  structure  anMogous  to  the  Malpighian  corpuscle  of 
the  permanent  kidney.  The  complexity  of  the  organ  is  in- 
creased by  the  development  of  secondary  tubules  and  Mal- 
pighian corpuscles  connected  with  those  first  formed. 

-  The  horizontal  or  transverse  tubules  of  the  W^olffian  body 
are  divisible  into  an  anterior  or  upper  series,  distinguished  as 
the  sexual  segment,  and  a  lower  set  of  atrophic  tubules — 
atrophic  for  reasons  that  will  appear  hereafter.  In  certain 
vertebrates  that  are  of  higher  type  than  those  in  which  the 
pronephros  functionates,  such  as  adult  amphibians  and  fishes, 
the  Wolffian  body  persists  throughout  life  as  an  organ  of 
urinary  secretion.  In  birds  and  mammals,  however,  its 
functional  activity  is  but  temporary,  since  it  is  supplanted, 
before  the  end  of  fetal  life,  by  the  permanent  kidney.  In 
man  it  disappears  relatively  early,  retrogression  beginning  in 
the  eighth  week  and  the  Malpighian  bodies  having  almost 
disappeared  by  the  fifth  month.  The  presence  of  the  meso- 
nephros  as  a  temporarily  functionating  organ  in  birds  and 
mammals,  while  it  is  a  permanent  structure  in  certain  lower 


DEVELOPMENT  OF  THE  KIDNEY  AND    URETER.     217 

members  of  the  vertebrate  series,  exemplifies  the  embryo- 
logical  principle  elsewhere  referred  to,  that  the  higher  types 
pass  through  stages  during  their  development  that  are  per- 
manent in  some  of  the  forms  below  them  in  the  scale  of 
evolution. 

The  Metanephros  or  Permanent  Kidney. — While  the  Wolffian 
body  is  temporarily  performing  the  office  of  a  kidney,  a 
structure  is  evolving  from  its  caudal  extremity  which  is 
to  form  the  permanent  organ.  It  has  been  stated  that 
the  duct  of  the  Wolffian  body  opens  into  the  cloaca. 
From  this  cloacal  end  of  the  duct,  a  little  diverticulum  or 
evagination  (Fig.  104)  grows  forth  and  soon  lengthens 
into  a  tube  which  extends  head- 
ward  toward  the  position  of 
the  Wolffian  body.  The  upper 
or  anterior  end  of  the  tube 
branches  to  form  a  number  of 
smaller  tubes  (Fig.  107).  At 
the  same  time,  the  surrounding 
indifferent  mesodermic  tissue 
(Fig.  108)  is  altered  in  charac- 
ter, becoming  condensed  and  vas- 

1  rrii        1  T     n  1       £•  1  ^^*'-  107.— Diagram  to  show  exten- 

CUlai%  Ihe  bimd  end  ot  each  sion  and  branching  of  kidney  e  vagi- 
little  tu^e,  becoming  dilated  and     "^"o°  ^"^  separation  of  its  staik 

^     ^  ^  from  the  Wolffian  duct :  M,  primitive 

saccular,  is  in vaginated  by  Capil-      ureter;   p,   pelvis    of   ureter;    Wd, 

lary    blood-vessels.      This    trans-      Wolfiian  duct;  £^  bladder;  ^suro- 
•^  genital  sinus  ;  CI,  cloaca ;  G,  gut. 

formation     produces    the     Mal- 

pighian  corpuscles.  The  sacculated  end  of  each  tubule  is  now 
a  capsule  of  Bowman,  while  the  tuft  of  capillarSI||^veloped 
by  it  is  the  glomerulus.  It  should  be  borne  in  mind  that  the 
glomerulus  is  not  contained  within  the  cavity  of  the  tubule, 
but  that  the  dilated  end  of  the  latter,  still  a  shut  sac,  folds 
around  the  tuft  of  capillaries,  enclosing  it  within  a  two- 
layered  wall. 

The  tubules,  assuming  tortuous  and  convoluted  form, 
owing  to  excessive  growth  in  length,  constitute  the  urin- 
iferous  tubules  of  %ie  adult  kidney.  According  J:o  Minot, 
the  capsule  of  the  kidney,  which  is   produced  by  the  sur- 


218  TEXT-BOOK  OF  EMBRYOLOGY. 

rounding  mesodermic  tissue,  exercises  an  important  influence 
upon  the  form  of  the  developing  tubules,  the  resistance  it 
offers  to  their  centrifugal  extension  being  the  determining 
factor  in  producing  the  foldings  and  convolutions  of  the 
tubules.  The  progress  of  these  metamorphoses  is  such  that 
the  kidney  acquires  its  characteristic  features  by  the  end  of 
the  second  month  of  fetal  life.     The  original  canal — that  is, 

Mesodermic  tissue. 


UrcUr. 

Fig.  108. — Diagrammatic  representation  of  the  development  of  the  kidney  (after 

Gegenbaur). 

the  stalk  of  the  diverticulum — is  now  the  excretory  duct  of 
the  kidney,  the  ureter,  while  the  dilated  and  branched  upper 
part  of  the  latter  is  the  pelvis  of  the  kidney  with  its  calyces 
and  infundibula.  The  connective-tissue  stroma  and  capsule 
of  the  organ,  in  common  with  its  blood-vessels  and  lymphatics, 
are  products  of  the  surrounding  mesodermic  tissue.  It  is 
apparent,  therefore,  that  all  parts  of  the  kidney  and  its  duct, 
the  epithelium  as  well  as  the  connective  tissue,  are  derived 
from  the  mesoderm,  since  the  Wolffian  duct,  from  which  the 
tubules  and  the  ureter  with  their  epithelium  develop,  is  itself 
a  product  of  the  mesodermic  cells,  or  raesothelium,  lining 
the  body-cavity. 

THE  SUPRARENAL  BODIES. 

The  development  of  these  structures  is  still  involved  in 
some  obscurity.  Most  observers  agree  that  they  are  geneti- 
cally connected  with  the  mesonephros  or  primitive  kidney, 
but  whether  the  organs,  as  represented  in  the  adult  condition, 


THE  SUPRARENAL  BODIES.  219 

are  derived  solely  from  this  source  is  not  as  yet  definitely 
made  out. 

The  origin  of  the  suprarenal  body  is  variously  ascribed  by 
different  investigators — 1,  to  a  group  of  connective-tissue 
cells  found  at  the  upper  end  of  the  primitive  kidney ;  2,  to 
the  germinal  epithelium  of  the  anterior  or  head-end  of  the 
genital  ridge ;  and  3,  to  epithelial  outgrowths  from  the 
mesonephros  or  primitive  kidney.  The  last  named  is  re- 
garded by  Hertwig  and  by  Minot  as  being  the  most  probable 
source.  The  epithelial  outgrowths  referred  to  bud  forth  from 
the  glomeruli  of  the  primitive  kidney  and  divide,  each  into 
two  branches,  one  of  which  grows  into  the  indifferent  sexual 
gland  to  produce  a  part  of  its  structure  (as  described  in  the 
sections  on  the  ovary  and  on  the  testis),  while  tiie  other  turns 
dorsad  and  spreads  out  to  form  eventually  the  cortical  part 
of  the  suprarenal  body. 

The  origin  of  the  medullary  part  of  the  organ  is  also  a 
disputed  point.  On  the  dorsal  side  of  the  primitive  kidney 
chains  of  small  cells  grow  forth  from  the  embryonic  ganglia 
of  the  sympathetic  system.  These  latter  become  surrounded 
by  the  cells  derived  from  the  mesonephros,  as  well  as  by 
embryonic  connective-tissue  cells,  and  are  divided  into  small 
groups.  According  to  Hertwig,^  who  follows  Balfour,  Braun, 
and  Kolliker,  the  cells  budding  from  the  sympathetic  ganglia 
produce  the  medullary  part  of  the  organ.  Minot,  on  the 
contrary,  believes  that  these  cells  later  entirely  disappear  and 
take  no  part  in  the  formation  of  the  medulla. 

In  the  early  stages  of  fetal  life,  the  suprarenal  body  is  rela- 
tively much  larger  than  in  the  adult  condition,  and  is  situated 
chiefly  on  the  ventral  surface  of  the  kidney.  At  about  the 
third  month  it  begins  to  assume  more  nearly  its  normal 
position. 

The  account  of  the  development  of  the  bladder  and  of  the 
urethra  may  be  deferred  until  the  evolution  of  the  internal 
sexual  system  shall  have  been  considered. 

^  Entwickelungsgeschichte,  fifth  edition,  Jena,  1896. 


220  TEXT-BOOK  OF  EMBRYOLOGY. 

THE    DEVELOPMENT    OF    THE    INTERNAL    GENERATIVE 

ORGANS. 

The  Indifferent  Type. — The  internal  generative  organs  of 
both  sexes,  in  the  course  of  their  development,  pass  through  a 
stage  in  which  there  is  to  be  found  no  distinction  of  sex. 
This  stage  is  designated,  therefore,  the  indifferent  type  of 
sexual  apparatus. 

While  the  Wolffian  body  is  attaining  its  full  development, 
there  appears  in  its  vicinity  a  tube,  the  duct  of  Miiller  (Plate 
VII.,  Fig.  1),  which  lies  parallel  with,  and  to  the  outer  side 
of,  the  Wolffian  duct.  The  exact  mode  of  origin  of  the  duct 
of  Miiller  has  not  as  yet  been  definitely  made  out.  Accord- 
ing to  one  view,  its  upper  or  cephalic  portion  is  produced 
by  an  invagination  of  the  mesothelium  of  the  body-cavity, 
while  the  remaining  lower  segment  results  from  a  fission 
or  longitudinal  division  of  the  Wolffian  duct.  In  what- 
ever way  the  duct  may  be  formed,  its  lower  or  caudal 
end  opens  into  the  cloaca,  which  receptacle  receives  also 
the  termination  of  the  Wolffian  duct.  The  upper  end 
of-  the  duct  maintains  a  communication  with  the  body- 
cavity  or  ccelom  by  means  of  an  expanded  funnel-shaped 
mouth.  Its  lower  segment  is  closely  associated  with  its  fel- 
low and  with  the  Wolffian  ducts,  forming  thus  the  genital 
cord.  The  function  of  this  canal  in  lowly  organized  ani- 
mals— that  of  receiving  from  the  body-cavity  the  female 
genital  products,  the  ova,  and  evacuating  them  from  the 
body — foreshadows  its  subsequent  metamorphosis  in  most 
vertebrates. 

While  the  duct  of  Miiller  is  forming,  the  mesothelial 
cells  overlying  that  part  of  the  free  surface  of  the  Wolf- 
fian body  which  looks  toward  the  median  plane  and 
somewhat  forward,  its  ventro-mesial  aspect,  undergo  multi- 
plication and  thickening  (Fig.  109,  a),  forming  an  elongated 
swelling  or  ridge.  This  is  known  as  the  genital  ridge,  which 
produces  a  projection  upon  the  wall  of  the  body-cavity.  The 
genital  ridge  is  still  further  thickened  by  the  proliferation  of 
the  mesodermic  tissue  {E)  beneath  the  mesothelial  cells.  The 
genital  ridges  of  the  human  fetus  appear  in  the  fifth  week. 


PLATE   Vll. 


Uretkra     M  /, 

Diagrammatic  representation  of  the  development  of  the  genito-urinary  system,  tlie 
Wolffian  body  and  its  derivatives  feeing  colored  red,  the  Mullerian  duct  and  its  de- 
rivatives, green :  1,  inditferent  type ;  2,  indifferent  type,  later  stage,  the  Wolffian  and 
Miillerian  ducts  and  the  primitive  ureter  now  opening  into  the  urogenital  sinus ; 
3,  male  type,  lower  ends  of  Mullerian  ducts  fused  to  form  the  sinus  pocularis;  4. 
female  type. 


THE  INTERNAL   GENERATIVE  ORGANS. 


221 


Further  diiFerentiation  of  the  genital  ridge  results  in  its 
transformation  into  the  so-called  indifferent  sezual  gland  (Plate 
VII.,  Fig.  1),  a  structure  common  to  both  sexes  at  this  stage. 
The  essential  feature  of  this  process  is  that  the  thickened 
mesothelial  cells  overlying  the  genital  ridge  become  modified 
in  character  and  penetrate  the  ridge  in  the  form  of  cords  or 
strands  of  cells.     These  mesothelial  elements  were  called  by 


Fig.  109.— Cross-section  through  the  mesonephros,  the  fundament  of  the  Miil- 
lerian  duct,  and  the  sexual  gland  of  a  chick  of  the  fourth  day  (after  Waldeyer), 
magnified  100  diameters:  m,  mesentery;  L,  somatopleure ;  a',  the  region  of  the 
germinal  epithelium  from  which  the  Miillerian  duct  (s)  has  been  invaginated ;  a, 
thickened  part  of  the  germinal  epithelium,  in  which  the  primary  sexual  cells,  C 
and  0,  lie  ;  E,  modified  mesenchyme  out  of  which  the  stroma  of  the  sexual  gland 
is  formed ;  WK,  mesonephros ;  y,  mesonephric  duct. 


Waldeyer  the  germinal  epithelium,  because,  after  their  exten- 
sion into  the  interior  of  the  ridge  or  gland,  they  give  rise  to 
the  germ-cells,  namely,  the  ova  or  the  spermatozoa  as  the  case 
may  be.  The  cell-cords  include  two  kinds  of  elements,  the 
smaller  mesothelial  cells  and  the  primitive  sexual  cells,  which 
latter  are  larger  and  less  numerous  than  the  mesothelial  cells 
and  have   large  nucleolated   nuclei.      The  primitive  sexual 


222  TEXT-BOOK   OF  EMBRYOLOGY. 

or  seminal  cells,  or  primitive  ova,  are  so  called  because  it  has 
been  assumed  that  they  develop  either  into  the  ova  or  the 
seminal  filaments  according  to  the  sex  of  the  embryo.  The 
cell-cords  have  been  seen  in  the  indifferent  gland  of  the 
human  embryo  as  early  as  the  fifth  week.  At  this  time, 
although  there  are  no  gross  sexual  distinctions  recognizable, 
it  is  possible  to  determine  from  the  histological  characters  of 
the  organ  whether  it  is  to  be  a  testis  or  an  ovary,  the  large 
sexual  cells  being  far  less  numerous  relatively  in  the  former 
case  than  in  the  latter  (Nagel). 

The  indifferent  sexual  gland  comes  into  an  especially  close 
relation  with  tlie  upper  or  sexual  series  of  the  mesonephros 
or  Wolffian  body  (Plate  VII.,  Fig.  1),  the  significance  of  which 
fact  will  appear  later. 

The  elements  of  the  indifferent  stage  of  the  sexual  ap- 
paratus are,  therefore,  the  indifferent  sexual  gland,  the  WoMan 
duct,  and  the  duct  of  MiiUer  (Plate  VIL,  Figs.  1,  2).  From 
this  asexual  stage,  either  the  male  or  the  female  type  is  pro- 
duced by  the  metamorphosis  of  the  indifferent  glands  into 
the  testicles  or  the  ovaries  and  the  formation  of  ducts  to 
provide  for  the  escape  of  the  sexual  elements,  the  spermatozoa, 
or  the  ova,  produced  by  them.  7^ 

The  Male  Type  of  Sexual  System. — The  differentiation  of 
the  indifferent  sexual  system  into  the  male  type  is  effected 
by  the  further  development  of  some  parts  and  the  atrophy 
or  the  arrested  growth  of  others. 

The  testicle  has  a  double  origin,  since  the  proper  secretory 
part  of  the  organ  is  produced  by  the  metamorphosis  of  the 
indifferent  sexual  gland,  while  its  system  of  efferent  ducts  is 
furnished  by  the  Wolffian  body.  Mention  has  been  made 
of  the  cell-cords  of  the  indifferent  sexual  glands  and  of  their 
origin  from  the  mesothelium  of  the  body-cavity,  and  also  of 
the  fact  that  they  consist  of  the  smaller  mesothelial  cells 
and    the    larger  and   less   numerous  primitive  sexual  cells. 

The  mesothelial  cells  increase  in  number  and  become  so 
grouped  as  to  form  cylindrical  masses  known  as  sexual  cords, 
each  of  which  includes  some  of  the  primitive  seminal  or 
sexual  cells.     By  the  ingrowth  of  connective  tissue  from  the 


THE  INTERNAL   GENERATIVE  ORGANS.  223 

surrounding  mesoderm,  the  sexual  cords  are  divided  into 
roundish  masses,  each  of  which  is  made  up  of  many  of  the 
smaller  elements  and  a  less  number  of  the  large  seminal 
cells.  These  follicle-like  masses  become  hollowed  out  to 
form  the  seminal  ampullae,  which  afterward,  by  undergoing 
great  increase  in  length,  are  transformed  into  the  seminiferous 
tubules.  During  fetal  life,  however,  and  even  to  the  period 
of  puberty,  the  "  tubules  "  remain  solid  cords  of  cells.  The 
exact  relation  of  the  two  kinds  of  cells  to  the  production  of 
the  spermatozoa,  whether  or  not  the  small  cells  give  rise  to 
Sertolli's  columns,  and  the  primitive  seminal  cells  to  the 
spermatozoa,  is  still  involved  in  obscurity. 

Spermatogenesis,  or  the  development  of  the  spermatozoa 
from  the  cells  that  line  the  seminiferous  tubules  of  the  func- 
tionating testicle,  has  been  considered  in  Chapter  I. 

While  the  sexual  cords  are  being  transformed  into  the 
cylinders  that  become  the  seminiferous  tubules,  the  surround- 
ing mesoderm ic  tissue  penetrates  the  genital  gland  and  forms 
the  connective-tissue  septa  that  constitute  the  stroma  of  the 
organ  and  divide  it  into  lobules.  At  the  same  time,  also, 
marked  changes  occur  in  the  Wolffian  body.  From  that  part 
of  this  structure  which  has  been  referred  to  as  the  sexual 
series  of  transverse  Wolffian  tubules,  cords  of  cells  grow 
forth  and  penetrate  the  genital  gland,  their  ends  fusing  with 
the  primitive  seminiferous  tubules.  The  conversion  of  these 
cell-cords  into  tubes  furnishes  the  initial  part  of  the  system 
of  excretory  ducts  of  the  testicle,  namely,  the  vasa  recta,  and 
the  rete  testis.  Somewhat  later,  in  the  twelfth  week,  the 
rete  testis  is  extended  to  form  the  vasa  efferentia,  and  still 
later,  in  the  fourth  and  fifth  months,  the  efferent  vessels 
lengthen  and  become  tortuous,  producing  thereby  the  coni  vas- 
culosi  or  head  of  the  epididymis  (Plate  YIL,  Fig.  3 ;  Fig.  110). 
The  upper  part  of  the  Wolffian  duct  develops  into  a  con- 
voluted tube  which  constitutes  the  body  and  tail  of  the  epididy- 
mis, while  the  lower  portion  becomes  the  vas  deferens,  thus 
completing  the  system  of  canals  provided  for  the  escape  of 
the  spermatozoa.  Near  the  caudal  end  of  the  Wolffian  duct, 
a  little  pouch-like  evagination  grows  from  its  wall  and  becomes 


224 


TEXT-BOOK  OF  EMBRYOLOGY. 


Fig.  110.— Internal  generative  or- 
gans of  a  male  fetus  of  about  fourteen 
weeks  (Waldeyer) :  t,  testicle;  e,  epi- 
didymis; w',  Wolffian  duct;  w,  lower 
part  of  Wolffian  body ;  g,  gubernacu- 
lum  testis. 


the  seminal  vesicle,  the  lower  end  of  the  duct,  below  the  orifice 
of  the  seminal  vesicle,  being  the  ejaculatory  duct.     Since  the 

Wolffian  duct  terminates  in  the 
cloaca,  and  since  the  anterior 
part  of  the  cloaca  corresponds 
to  a  portion  of  the  later  ure- 
thra, the  termination  of  the 
ejaculatory  duct  in  the  pros- 
tatic part  of  the  urethra  is 
accounted  for.  Thus  it  will 
be  seen  that  while  the  secret- 
ing part  of  the  testicle  results 
from  the  transformation  of  the 
indiiferent  genital  gland,  the 
secretory  cells  having  their 
origin  in  the  germinal  epithe- 
lium, the  complicated  system 
of  ducts  with  which  it  is  provided  is  furnished  by  the  meso- 
nephros  or  Wolffian  body. 

The  series  of  tubules  connected  with  the  upper  extremity 
of  the  Wolffian  duct,  the  remnant  of  the  pronephros  or  head- 
kidney,  frequently  persists  as  a  little  pedunculated  sac  at- 
tached to  the  upper  part  of  the  epididymis ;  it  is  known  as 
the  stalked  hydatid  and  sometimes  also  as  the  hydatid  of 
Morgagni.  The  posterior  or  lower  set  of  Wolffian  tubules 
likewise  give  rise  to  an  atrophic  structure,  the  paradidymis  or 
organ  of  Giraldes,  which  consists  of  a  series  of  short  tubes 
closed  at  both  ends,  lying  among  the  convolutions  of  the  tail 
of  the  adult  epididymis  (Plate  YII.,  Fig.  3). 

The  duct  of  Muller  remains  atrophic,  in  the  male,  through- 
out its  entire  extent,  and  in  fact,  with  the  exception  of  its 
two  extremities,  it  usually  altogether  disappears.  Its  upper 
extremity  persists  as  a  small  vesicle,  the  unstalked  or  sessile 
hydatid,  attached  to  the  upper  aspect  of  the  testicle.  The 
lower  extremity  of  the  duct,  uniting  with  its  fellow,  becomes 
converted  into  the  sinus  pocularis  or  uterus  masculinus  of  the 
prostate  gland  (Plate  VII.,  Fig.  3).     If  the  intervening  part 


THE  INTERNAL  GENERATIVE  ORGANS.  225 

of  the  tube  persists  to  post-natal  life  and  remains  patulous, 
it  is  known  as  the  duct  of  Rathke. 

The  change  of  location  which  the  testicle  undergoes  is  a 
conspicuous  feature  of  its  development.  To  understand  this 
clearly,  it  is  necessary  to  recall  the  relation  of  the  meso- 
nephros  and  the  genital  gland  to  the  peritoneum.  Since 
both  of  these  bodies  originate  from  the  cells  of  the  outer 
wall  of  the  body-cavity,  or,  in  other  words,  from  what  be- 
comes the  parietal  peritoneum,  necessarily  they  lie  between 
the  body-wall  and  the  parietal  peritoneum — that  is,  behind 
the  peritoneal  cavity.  With  the  increase  in  size  of  these 
structures,  they  project  toward  the  peritoneal  cavity,  the 
peritoneum  passing  over  them  and  forming  a  "  mesentery," 
which  anchors  them  to  the  posterior  wall  of  the  abdomen. 
In  the  case  of  the  testicle,  this  peritoneal  fold  or  "  mesentery  " 
is  called  the  mesorchium ;  in  the  case  of  the  ovary,  the  meso- 
varium.  It  is  prolonged  upward  to  the  diaphragm  as  the 
diaphragmatic  ligament  of  the  primitive  kidney,  and  down- 
ward to  the  inguinal  region  as  the  inguinal  ligament  of  the 
primitive  kidney  (Fig.  106),  since  this  latter  organ  is  the 
largest  constituent  of  the  projecting  mass.  When  the  primi- 
tive kidney  has  disappeared  as  such,  the  inguinal  ligament 
mentioned  seems  to  connect  the  ovary  or  testicle  with  the 
inguinal  region  of  the  abdominal  wall. 

The  inguinal  ligament  contains  between  its  folds  connec- 
tive tissue  and  unstriped  muscular  fibers.  These  become  the 
gubernaculum  testis  in  the  male  or  the  round  ligament  of  the 
uterus  in  the  female.  As  the  body  of  the  fetus  continues  to 
grow  while  the  tissues  of  the  ligament  remain  stationary  or 
grow  less  rapidly,  the  testicle  is  gradually  displaced  from  its^ 
position  at  the  side  of  the  lumbar  spine,  and  by  the  third 
month  reaches  the  false  pelvis.  In  the  fifth  and  sixth  months 
it  is  in  contact  with  the  anterior  abdominal  wall,  near  the 
inner  abdominal  ring.  In  the  eighth  month  it  enters  the 
inguinal  canal,  and  near  the  end  of  the  ninth  month,  shortly 
before  birth,  it  leaves  the  inguinal  canal  and  enters  the 
scrotum.^ 

^  Non-descent  of  the  testicles,  with  consequent  emptiness  and  flabbiness 
15 


226  TEXT-BOOK  OF  EMBRYOLOGY. 

Before  the  testicle  leaves  the  abdominal  cavity,  the  parietal 
peritoneum  pouches  through  the  inguinal  canal  into  the 
scrotum,  the  protruded  part  being  the  processus  vaginalis. 
Since  the  testicle  is  from  the  first  behind  the  parietal  peri- 
toneum, it  passes  into  the  scrotum  behind  the  vaginal  process, 
the  latter  then  folding  around  it  as  a  shut  sac  of  two  layers. 
Subsequently  the  connection  of  the  sac,  now  the  tunica  vagi- 
nalis testis,  with  the  abdominal  peritoneum  is  reduced  to  a 
slender  strand  of  tissue  lying  in  front  .of  the  spermatic  cord.^ 

The  testicle  necessarily  carries  with  it,  in  its  descent,  its 
blood-vessels,  the  spermatic  artery  and  vein ;  its  duct,  the 
vas  deferens ;  as  well  as  its  nerves  and  lymphatic  vessels ; 
and  these  structures  collectively  constitute  the  spermatic  cord. 

The  Female  Type  of  Sexual  System. — While  the  indifferent 
sexual  gland,  in  the  development  of  the  male  generative  sys- 
tem, undergoes  metamorphosis  into  the  testicle,  it  becomes, 
in  the  evolution  of  the  female  type,  so  altered  as  to  consti- 
tute the  ovary ;  and  while  the  Wolffian  tubules  and  the 
Wolffian  body  become  in  the  male  the  system  of  excretory 
ducts  of  the  testicle,  they  produce  in  the  female  merely 
atrophic  structures.  On  the  other  hand,  the  duct  of  Miiller, 
which  gives  rise  in  the  male  to  atrophic  appendages,  forms  in 
the  female  type  the  Fallopian  tube  and,  by  fusing  with  its 
fellow  of  the  opposite  side,  the  uterus  and  the  vagina. 

The  ovary  results  from  alterations  in  the  structure  of  the 
genital  gland  analogous  to  those  that  occur  in  the  evolution 
of  the  testicle.  The  special  features  of  these  changes  are 
better  understood,  however,  than  are  those  of  the  testicle. 
As  in  the  case  of  the  development  of  the  testicle,  the  meso- 
thelial  cells  on  the  peritoneal  surface  of  the  genital  ridge 
become  thickened,  these  altered  cells  constituting  the  germinal 

of  the  scrotum,   is  designated  cryptorchism  (hidden  testes).     The  presence 
of  but  one  testicle  in  the  scrotum  is  called  monorchism. 

^Occasionally  it  happens  that  the  funicular  process  of  the  tunica  vagi- 
nalis— that  is,  tlie  stalk  of  the  sac,  remains  patulous  throughout  its  entire 
extent,  a  condition  which  allows  of  the  easy  and  sudden  protrusion  of  a 
segment  of  the  bowel  into  the  cavity  of  the  tunica  vaginalis,  constituting 
the  so-called  congenital  hernia.  Or  the  funicular  process  may  close  only 
at  one  or  the  other  end,  giving  rise  to  other  varieties  of  hernia. 


THE  INTERNAL   GENERATIVE  ORGANS. 


227 


epithelium  (Fig.  109).  Coincidentally,  the  primitive  connec- 
tive tissue — mesodermic  tissue — underlying  the  germinal 
epithelium  proliferates,  contributing  to  the  thickness  of  the 
genital  ridge.  By  the  sixth 
or  seventh  week,  the  ger- 
minal epithelium  consists  of 
several  strata  of  cells,  groups 
of  which  begin  to  penetrate 
the  underlying  mesodermic 
tissue  in  the  form  of  cord- 
like processes  (Fig.  112, 
e,  sch).  The  indifferent 
mesodermic  tissue  at  the 
same  time  increases  in  quan- 
tity, in  turn  penetrating 
between  the  groups  of  ad- 
vancing cells,  so  that  what 
takes  place  might  be  de- 
scribed as  a  mutual  inter- 
growth.  The  presence  of 
the  growing  connective  tis- 
sue accentuates  the  grouping  of  the  cells  into  cylindrical 
masses.  These  latter  are  the  sexual  cords  or  egg-columns 
(Pfliiger's  egg-tubes).  They  contain  two  special  kinds  of 
cells,  the  large  sexual  cells  or  primitive  ova  (Fig.  112,  ite), 
and  the  smaller  but  more  numerous  mesothelial  cells. 
The  connection  of  the  sexual  cords  with  the  germinal  epi- 
thelium is  much  more  obvious  in  this  case  than  in  the 
case  of  the  developing  testicle,  and  the  primitive  sexual  cells 
are  much  more  abundant.  The  egg-columns,  surrounded  by 
young  connective  tissue,  constitute  the  nucleus  of  the  cortical 
part  of  the  future  ovary.  This  mass  is  later  sharply  marked 
off  from  the  free  or  peritoneal  aspect  of  the  gland,  the  region 
of  the  germinal  epithelium,  by  a  zone  of  proliferating  meso- 
dermic cells  which  become  the  tunica  albuginea  of  the  ovary. 
An  important  change  now  takes  place  in  the  egg-columns ; 
the  primitive  ova,  or  large  sexual  cells,  increase  in  size,  their 
nuclei  becoming  especially  well  developed,  while  the  small 


Fig.  111. — Internal  organs  of  a  female 
fetus  of  about  fourteen  weeks  (Waldeyer) : 
o,  ovary ;  e,  epoophoron  or  parovarium ; 
w',  Wolffian  duct;  m,  Miillerian  duct;  w, 
lower  part  of  the  Wolffian  body. 


228 


TEXT-BOOK  OF  EMBRYOLOGY. 


mesothelial  cells  become  smaller  and  less  conspicuous.  Sev- 
eral of  the  large  cells  fuse  into  a  single  mass  of  protoplasm, 
■  while  one  of  the  nuclei  outstrips  the  others  in  growth  and, 
with  the  surrounding  zone  of  protoplasm,  becomes  the  ovum. 
Each  egg-column  is  now  broken  up  into  several  groups  of 
cells  by  the  penetration  of  connective  tissue,  each  group  (Fig. 
112,  e,  sch')  containing  a  single  ovum  but  many  of  the  smaller 


e.sd' 


Fig.  112.— Part  of  sagittal  section  of  an  ovary  of  a  child  just  born  (after  Wal- 
deyer).  Highly  magnified:  ke,  germinal  epithelium;  e,  sch,  Pfliiger's  egg-tubes; 
u€,  primitive  ova  lying  on  the  germinal  epithelium ;  e,  sch',  long  Pfliiger's  tubes,  in 
process  of  being  converted  into  follicles  ;  ei,  b,  egg-balls  (nests),  likewise  in  process 
of  being  resolved  into  follicles ;  /,  youngest  follicle  already  isolated :  gg,  blood- 
vessels. In  the  tubes  and  egg-nests  the  primordial  eggs  are  distinguishable  from 
the  smaller  epithelial  cells,  the  future  follicular  epithelium. 


cells.  These  groups  are  the  young  G-raafian  follicles  of  the 
ovary  (/).  The  enveloping  zone  of  connective  tissue  becomes 
the  theca  of  the  follicle,  while  the  single  large  cell  constitutes 
the  ovum,  and  the  smaller  cells  are  the  membrana  granulosa. 
At  first  the  granulosa  cells  surround  the  ovum  as  a  single 
layer  of  flattened  cells  which  gradually  assume  the  columnar 
type  and  become  so  numerous  as  to  form  many  layers.  They 
secrete  a  fluid,  the  liquor  folliculi,  which  crowds  the  ovum  to 
one  side  of  the  follicle  where  it  is  enveloped  by  a  special 
group  of  granulosa-cells,   the  discus  proligerus   (Fig,    113). 


THE  INTERNAL   GENERATIVE  ORGANS. 


229 


The  question  of  the  origin  of  the  follicular  cells  is  still  an 
unsettled  one,  though  it  seems  probable  that  they  are  derived 
from  the  cells  of  the  egg-columns,  and  Minot  believes  that 
they  are  probably  descended  from  the  primitive  ova. 

The  formation  of  new  Graafian  follicles,  and  consequently 
of  ova,  begins  in  the  deeper  part  of  the  ovary  and  advances 
toward  the  surface.     The  production  of  ova  and  follicles  is 


Fig.  113.— Section  of  human  ovary,  including  cortex  :  a,  germinal  epithelium 
of  free  surface ;  6,  tunica  albuginea ;  c,  peripheral  stroma  containing  immature 
Graafian  follicles  (d) ;  e,  well-advanced  follicle  from  whose  wall  membrana  granu- 
losa has  partially  separated ;  /,  cavity  of  liquor  foUiculi ;  g,  ovum  surrounded  by 
cell-mass  constituting  discus  proligerus  (Piersol). 

limited  to  the  fetal  stage  and  to  the  early  part  of  post-natal 
life,  their  formation  not  occurring,  according  to  Waldeyer, 
after  the  second  year. 

What  has  been  said  above  refers  to  the  development  of  the 
cortex  of  the  ovary.  The  medulla  is  produced  by  the  growth 
toward  the  egg-columns  of  cord-like  processes,  the  medullary 
cords,  from  the  epithelial  walls  of  the  Malpighian  corpuscles 
of  the  primitive  kidney  or  Wolffian  body,  the  cords  becoming 
surrounded  by  connective  tissue  and  forming  a  network.  The 
fetal  medullary  cords  are  represented  in  both  the  cortex  and 
the  medulla  of  the  mature  ovary  by  the  groups  of  interstitial 
cells  disposed  between  the  bundles  of  the  stroma-tissue. 


230  TEXT-BOOK  OF  EMBRYOLOGY. 

The  Oviducts,  the  Uterus,  the  Vagina. — The  system  of  pass- 
age-ways that  constitute  the  outlets  for  the  ova  and  the 
means  of  nourishing  tliem  and  evacuating  the  product  of 
gestation  from  the  body  in  the  event  of  impregnation — 
namely,  the  Fallopian  tubes,  the  uterus,  and  the  vagina — result 
from  the  metamorphosis  of  the  ducts  of  Miiller.  These  ducts, 
as  stated  above,  lie  along  the  dorsal  aspect  of  the  body-cavity, 
separated  from  it  by  the  parietal  peritoneum,  and  parallel 
with  the  primitive  spinal  column  (Plate  VII.).  The  probable 
method  of  their  formation  has  been  pointed  out  (p.  220). 
Near  the  lowe'r  (caudal)  end  of  the  body  they  approach  each 
other,  and  finally  unite  about  the  second  month  to  form  a 
single  duct  for  the  rest  of  their  extent  (Plate  VIL,  Fig.  4).  The 
upper,  ununited  parts  of  the  ducts  become  the  Fallopian  tubes 
or  oviducts,  while  the  lower  portions,  now  fused  into  one, 
become  the  uterus  and  the  vagina.  The  upper  end  of  each 
single  duct  expands  trumpet-like  to  form  the  fimbriated  ex- 
tremity of  the  Fallopian  tube. 

Until  the  fifth  month  there  is  no  distinction  between  the 
vagina  and  the  uterus,  the  two  being  represented  by  a 
single  sac-like  structure.  The  development  of  a  circular 
ridge  in  the  wall  of  the  sac  marks  the  division  between  the 
two  organs,  the  part  above  the  ridge  acquiring  thick 
muscular  walls,  while  the  part  below  it,  the  future  vagina, 
remains  thin-walled  and  more  capacious.  In  the  third 
month  the  uterus  is  bifid  at  its  upper  extremity,  a  condition 
which  is  permanent  in  some  animals  and  occasionally  in  the 
human  subject.^ 

The  Wolffian  duct,  which,  in  the  male,  becomes  metamor- 
phosed into  a  part  of  the  epididymis  and  the  vas  deferens, 
remains  undeveloped  in  the  female,  producing  merely  atrophic 
or  vestigial  structures  (Plate  VIL,  Fig.  4).  The  upper  series  of 
Wolffian  tubules,  the  remnant  of  the  pronephros,  frequently 

^  The  formation  of  the  uterus  aud  of  the  vagina  by  the  coalescence  of  two 
parallel  tubes  affords  an  explanation  of  the  uterus  bicornis  or  bifid  uterus  and 
of  the  condition  of  double  uterus  sometimes  met  with,  as  also  of  the  ppesence 
of  a  median  septum  in  the  vac/ina,  since  by  the  failure  of  union  of  the  two 
tubes  in  greater  or  less  degree  one  or  other  of  these  anomalies  would  result. 


THE  INTERNAL   GENERATIVE  ORGANS.  231 

persists,  as  in  the  male,  in  the  form  of  a  small  pedunculated  sac, 
the  stalked  hydatid  or  hydatid  of  Morgagni.  When  present,  it 
is  to  be  found  in  the  broad  ligament,  in  the  neighborhood  of 
the  outer  extremity  of  the  ovary.  The  middle  or  sexual 
series  of  the  Wolffian  tubules  with  the  adjacent  part  of  the 
Wolffian  duct,  which,  in  the  male  type,  develop  into  the 
epididymis,  become  in  the  female,  an  atrophic  structure 
known  as  the  epoophoron  or  parovarium,  or  organ  of  Eosen- 
miiller  (Fig.  111).  This  structure,  which  is  almost  con- 
stantly found  between  the  layers  of  the  broad  ligament  in 
close  proximity  to  the  ovary,  consists  of  a  larger  horizontal 
tube  representing  a  segment  of  the  Wolffian  duct,  and  of 
shorter  vertical  tubes  joining  this  at  a  right  angle  and  rep- 
resenting the  transverse  Wolffian  tubules.  The  lower  set 
of  small  Wolffian  tubules,  those  which,  in  the  male  become 
the  paradidymis,  give  rise  in  this  case  to  a  similar  atrophic 
body,  the  paroophoron.  This  is  also  situated  in  the  broad 
ligament,  usually  to  the  inner  side  of  the  ovary.  The 
Wolffian  duct,  with  the  exception  of  that  portion  of  it  that 
assists  in  the  formation  of  the  parovarium,  usually  entirely 
disappears.  Occasionally,  however,  it  persists  as  a  small 
canal  traversing  the  broad  ligament  close  to  the  uterus  and 
passing  on  the  dorsal  side  of  the  upper  part  of  the  vagina  to 
be  lost  upon  the  wall  of  the  latter  or,  more  rarely,  to  open 
near  the  urinary  meatus.  When  thus  persistent,  it  is  known 
as  the  duct  of  Gartner. 

The  change  of  position  of  the  ovaries  is  similar  to,  though 
less  marked  than,  that  of  the  testes.  The  inguinal  ligament  in 
the  female  (Plate  VII.)  extends  from  the  primitive  position 
of  the  ovaries  in  the  lumbar  region  of  the  abdominal  cavity 
to  the  groin,  Avhere  it  passes  through  the  abdominal  wall, 
traversing  the  inguinal  canal,  to  terminate  in  the  labium 
majus.  The  upper  part  of  this  ligament,  containing  invol- 
untary muscular  substance,  firmly  unites  witli  the  ovary.  In 
the  third  month  the  ovary  descends  to  the  lower  part  of  the 
abdominal  cavity  and  is  now  connected,  by  the  succeeding 
portion  of  the  inguinal  ligament,  with  the  uterus.  This  con- 
nection may  be  a  factor  in  the  final  change  of  position  of  the 


"232  TEXT-BOOK  OF  EMBRYOLOGY. 

ovary — that  is,  its  descent  into  the  true  pelvis.  The  part  of 
the  inguinal  ligament  that  passes  from  the  ovary  to  the  ute- 
rus is  the  permanent  ligament  of  the  ovary,  while  the  remain- 
ing portion,  which  passes  from  the  uterus  through  the  ingui- 
nal canal  to  the  labium  majus  of  the  vulva,  is  the  round  liga- 
ment of  the  uterus.  As  the  inguinal  ligament  perforates  the 
abdominal  wall,  a  small  diverticulum  of  peritoneum  goes 
with  it.  Normally  this  peritoneal  pouch  subsequently  be- 
comes obliterated.  Occasionally,  however,  it  persists  and 
then  constitutes  the  canal  of  Nuck.  Should  the  canal  of 
Nuck  be  present,  the  ovary  may  pass  into  or  through  it, 
thus  reaching  the  labium  majus.  A  patulous  canal  of  Nuck, 
as  in  the  case  of  a  patulous  funicular  process  of  the  tunica 
vaginalis  of  the  male,  may  permit  the  sudden  occurrence  of 
an  inguinal  hernia  in  the  female. 

The  account  of  the  development  of  the  external  genital 
organs  will  be  deferred  until  after  the  consideration  of  the 
formation  of  the  urinary  bladder  and  of  that  part  of  the 
urethra  that  originates  from  the  same  embryonic  structure. 


7  THE  BLADDER  AND  THE  PROSTATE  GLAND. 

As  stated  in  Chapter  V.,  the  urinary  bladder  and  a  part  of 
the  urethra  are  derived  from  the  intra-embryonic  portion  of 
the  allantois.  In  the  same  chapter  the  allantois  was  described 
as  a  sac  which  developed  as  a  pouching-out  of  the  ventral 
wall  of  the  gut-tract  near  its  caudal  end  (Plate  II., 
5  and  6).  The  sac  protrudes  from  the  still  widely  open 
abdominal  cavity,  enters  the  extra-embryonic  part  of  the 
body-cavity,  and  reaches  the  inner  surface  of  the  false 
amnion,  with  which  structure  it  intimately  unites  to  form 
the  true  chorion  (Plate  III.).  As  the  walls  of  the  abdomen 
gradually  close,  leaving  only  the  umbilical  aperture,  it  is, 
necessarily,  through  this  aperture  that  the  allantois  pro- 
trudes. 

We  have  seen  (p.  82)  what  becomes  of  the  extra-abdom- 
inal part  of  the  allantois — in  what  degree  it  contributes  to 
the  formation  of  the  placenta  and  of  the   umbilical   cord. 


THE  BLADDER  AND   THE  PROSTATE  GLAND.      233 

Obviously,  with  the  severing  of  the  umbilical  cord  after 
birth,  all  this  extra-embryonic  part  of  the  allantois  disap- 
pears, giving  rise  to  no  adult  organ. 

Its  intra-embryonic  portion  consists  of  a  tube  extending 
from  the  caudal  end  of  the  intestine  to  the  umbilicus  (Plate 
II.,  5  and  6).  As  early  as  the  second  month,  the  middle 
segment  of  this  tube  dilates  and  assumes  the  form  of  a  spindle- 
shaped  sac,  which  becomes  the  urinary  bladder  (Plate  VII.). 
The  part  of  the  tube  connecting  the  summit  of  this  sac  with 
the  umbilicus  remains  small,  gradually  loses  its  lumen,  and 
constitutes  in  the  adult  the  (usually)  impervious  cord  known 
as  the  urachus.  Should  the  cavity  of  the  urachus  persist  in 
its  entirety,  and  should  there  be  at  the  same  time  an  external 
opening  at  the  umbilicus,  the  condition  would  constitute  an 
umbilical  urinary  fistula.  The  proximal  part  of  the  allantois 
— that  is,  the  portion  intervening  between  the  bladder  and 
the  intestine — is  designated  the  sinus  urogenitalis,  while  the 
caudal  end  of  the  intestine,  which  is,  in  effect,  a  pouch  in 
which  both  the  allantois  and  the  intestine  terminate,  is  known 
as  the  cloaca  (Fig.  84).  The  urogenital  sinus  receives  the 
terminations  of  both  the  Miillerian  and  the  Wolffian  ducts 
(Plate  VII.). 

In  the  sixth  week  or  slightly  earlier,  there  appears  upon  the 
surface  of  the  body,  in  the  region  corresponding  to  the  position 
of  the  cloaca,  a  depression,  the  cloacal  depression  (Fig.  84), 
which  later,  except  in  man  and  the  higher  mammals,  meets 
the  cloaca,  and  thus  establishes  a  communication  between  it 
and  the  exterior.  In  the  Amphibia,  in  reptiles,  and  in  birds, 
as  also  in  the  lowest  mammals,  the  monotremes,  the  cloaca 
is  a  permanent  structure,  and  through  it,  in  these  groups  of 
animals,  not  only  the  fecal  matters  and  the  urine,  but  also 
the  genital  products,  the  spermatozoa  and  the  ova,  are  evacu- 
ated from  the  body.  In  all  mammals,  however,  with  the 
exception  of  the  monotremes,  the  cloaca  undergoes  division 
into  a  posterior  part  or  anal  canal  and  an  anterior  urogenital 
aperture.  This  division  is  brought  about  by  the  growth  of 
three  ridges  or  folds,  of  which  one  springs  from  each  side 
of  the  cloaca  and  one  from  the  point  of  union  of  the  uro- 


234  TEXT-BOOK   OF  EMBRYOLOGY. 

genital  sinus  and  the  intestine.  These  folds  coalesce  about 
the  eighth  ^  week  to  form  a  complete  septum,  which  continues 
to  thicken  antero-posteriorly  up  to  the  time  of  birth  and 
constitutes  the  perineum. 

It  will  be  remembered  that  the  ureters  originally  spring 
from  the  terminal  parts  of  the  Wolffian  or  mesonephric  ducts 
(Fig.  104).  Owing  to  alterations  brought  about  by  processes 
of  unequal  growth,  the  orifices  of  the  ureters  subsequently 
change  their  position  so  as  to  open  into  the  urogenital  sinus 
(Fig.  107),  and  still  later,  by  the  further  operation  of  the 
same  agency,  they  come  to  open  into  the  bladder  on  its  dor- 
sal wall,  thus  gradually  assuming  their  permanent  relations 
(PI.  VII.).  After  the  division  of  the  cloaca  the  urogenital 
sinus,  as  stated  above,  opens  independently  upon  the  surface 
of  the  body.  In  the  female  it  is  transformed  into  a  short 
tube,  the  urethra,  and  an  expanded  terminal  recess  or  fossa, 
the  vestibule  of  the  vulva  (PI.  VII.).  In  the  male  it  be- 
comes the  first  or  prostatic  part  of  the  urethra. 

In  the  twelfth  or  thirteenth  week,  the  future  prostatic  ure- 
thra acquires  very  thick  muscular  walls,  and  the  original 
epithelial  tube  pouches  out  into  the  muscular  tissue  in  the 
form  of  little  sacs,  the  lining  cells  of  which  assume  the  char- 
acters of  secreting  epithelium.  In  this  way  is  produced  the 
aggregation  of  muscular  and  glandular  tissue  known  as  the 
prostate  gland.  This  is  a  well-developed  structure  by  the 
fourth  or  fifth  month  (Tourneux).  The  recess  in  the  floor 
of  this  part  of  the  urethra,  the  sinus  pocularis  or  uterus  mas- 
culinus,  has  been  previously  referred  to  as  the  homologue  of 
the  uterus,  being  the  persistent  caudal  extremities  of  the 
ducts  of  Miiller  (Plate  VII.). 

THE  EXTERNAL  ORGANS  OF  GENERATION. 

In  the  early  stages  of  the  development  of  the  external 
genital  organs  no  sexual  distinctions  are  apparent. 

Reference  has  been  made  to  the  cloacal  depression  as  a 
superficial  fossa  which  makes  its  appearance  at  the  caudal  end 
of  the  body  of  the  embryo  in  the  sixth  week  (Fig.  84).  At 
^  Fourteenth  week,  according  to  Minot. 


THE  EXTERNAL   ORGANS  OF  GENERATION. 


235 


about  the  same  period  an  encircling  elevation,  the  genital  ridge 
(Fig.  114:,  A,  4),  is  seen  to  surround  this  depression.  Within 
the  genital  ridge,  at  the  anterior  part  of  the  cloacal  fossa,  a 
small  tubercle,  the  genital  eminence,  appears  at  the  same  time. 


Fig.  114.— Four  successive  stages  of  development  of  the  external  genital  organs 
(indifferent  type)  of  the  human  fetus  of  24  to  34  mm.  (0.95  to  1.35  inch)  (Tourneux) : 
1,  genital  eminence  or  tubercle ;  2,  glans ;  3,  genital  groove ;  4,  genital  ridge ;  b, 
cloacal  depression ;  6,  coccygeal  eminence. 

On  the  under  aspect  of  the  genital  eminence  there  is  soon 
distinguishable  the  genital  groove  (Fig.  114,  3),  which  appears 
as  if  a  continuation  of  the  fissure-like  cloacal  depression  (5), 
and  the  groove  very  shortly  becomes  flanked  by  two  ridges, 
the  genital  folds,  one  on  each  side. 

The  genital  eminence  becomes  the  penis  or  the  clitoris, 
according  to  the  sex  of  the  fetus.  It  very  early  acquires  a 
knob-like  extremity  (2)  which  is  the  beginning  of  the  glans 
penis  or  of  the  glans  clitoridis,  as  the  case  may  be.  Further 
development  of  the  glans  is  brought  about  by  the  appearance 
of  a  partially  encircling  groove  which  serves  to  differentiate 
it  from  the  body  of  the  organ. 

At  this  stage  of  development,  the  rudimentary  organs,  as 


236 


TEXT-BOOK  OF  EMBRYOLOGY. 


described  above,  are  precisely  alike  in  the  two  sexes.  Early 
in  the  third  month — about  the  ninth  week — sexual  distinc- 
tions begin  to  become  manifest.  Since  the  female  organs 
exhibit  the  less  degree  of  deviation  from  the  early  indifferent 
form,  they  will  be  first  considered. 

The  External  Genital  Organs  of  the  Female. — The  sexually 
indifferent  genital  eminence  which,  as  we  have  seen,  presents 


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•'IS  f 


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lu 


.  i 


J' 

I  ■ 


Fig.  115. — Four  successive  stages  of  development  of  the  external  genital  organs 
of  the  human  female  fetus  (Tourneux) :  1,  clitoris ;  2,  glans  clitoridis ;  3,  urino- 
genital  fissure;  4,  labia majora;  5,  anus;  6,  coccygeal  eminence;  7,  labia  minora. 

even  by  the  end  of  the  second  month  a  rudimentary  glans  and 
an  indication  of  a  prepuce,  elongates  somewhat  and  becomes 
the  clitoris.  The  genital  folds  bounding  the  genital  groove 
on  the  under  surface  of  the  genital  eminence  (Figs.  114  and 


THE  EXTERNAL   ORGANS  OF  GENERATION        237 

115,  A)  never  unite  with  each  other  as  they  do  in  the  male, 
but  become  prolonged  in  the  direction  of  the  future  anus  and 
constitute,  by  the  fourth  month,  the  lateral  boundaries  of  the 
orifice  of  the  urogenital  sinus,  or,  in  other  words,  of  the  ves- 
tibule of  the  vagina  (Fig.  115,  3).  These  folds,  continuous 
over  the  dorsum  of  the  clitoris  with  its  rudimentary  prepuce, 
are  the  nymphae  or  labia  minora  (Fig.  115,  D,  7)  of  the  fully- 
formed  state.  The  masses  of  erectile  tissue  in  close 
relation  with  each  labium  minus,  the  pars  intermedialis 
and  the  bulbus  vestibuli,  are  the  homologues  respectively 
of  a  lateral  half  of  the  male  corpus  spongiosum  and  its 
bulb.  The  genital  ridge,  which,  from  the  first,  encircles 
the  genital  eminence  and  the  cloacal  depression,  and,  con- 
sequently, the  later  clitoris  and  the  aperture  of  the  sinus 
urogenitalis,  increases  greatly  in  thickness.  The  part  of  it 
situated  on  the  ventral  side  of  the  clitoris  becomes  the  mens 
veneris,  while  the  lateral  parts  of  the  ridge  become  the  labia 
majora  of  the  vulva.  The  several  parts  of  the  female  geni- 
talia develop  to  such  a  degree  during  the  fourth  month  that 
their  sexual  characters  at  this  time  are  well  marked. 

The  reader  is  again  reminded  that  in  the  stage  when  the 
cloaca  is  present,  the  Miillerian  ducts  terminate  in  the  sinus 
urogenitalis  (Plate  VIL).  As  previously  stated,  the  sinus  uro- 
genitalis becomes  the  female  urethra,  its  terminal  portion 
expanding  into  the  vestibulum  vaginae.  The  openings  of  the 
Miillerian  ducts  fall  within  this  latter  vestibular  region  of 
the  sinus.  The  lower  portion  of  the  two  ducts  by  this  time, 
however,  have  fused  to  form  the  uterus  and  the  vagina,  and 
hence  is  established  the  permanent  relationship  of  these  parts 
— that  is,  the  opening  of  the  vagina  and  the  urethra  by  sepa- 
rate orifices  into  the  vestibule. 

The  formation  of  the  hjnnen  begins  in  the  fifth  month  as  a 
little  crescentic  fold  at  the  posterior  margin  of  the  aperture 
of  the  vagina. 

The  glands  of  Bartholin  develop  as  evaginations  of  the 
wall  of  the  vestibular  region  of  the  urogenital  sinus. 

The  Male  External  Genitals. — The  male  external  organs 
represent  a  farther   stage  of  development  than  the  corre- 


238 


TEXT-BOOK  OF  EMBRYOLOGY. 


spoil  ding  female  parts.  The  genital  eminence  elongates  rather 
rapidly,  its  length  in  the  tenth  week  being  1.5  mm.  The 
knob-like  extremity  becomes  better  marked  and  constitutes 
the  glans  penis,  while  the  integumentary  fold  that  partially 
encircles  the  latter  assumes  more  distinctive  character  as  the 
prepuce.  This  fold  gradually  advances  over  the  glans  and 
adheres  to  it,  the  adhesion  persisting  until,  or  shortly  after, 
birth.  All  of  the  rudimentary  penis,  exclusive  of  the  glans 
and  of  the  genital  folds,  becomes  the  corpora  cavernosa  of 
the  adult  organ.     The  characteristic  structure  of  the  corpora 


X 


J 


Fig.  116.— Four  successive  stages  of  development  of  the  human  male  fetus 
(Tourneux) :  1 ,  penis ;  2,  glans ;  3,  genital  groove ;  4,  scrotum ;  5,  anus ;  6,  coccygeal 
eminence;  7,  perineoscrotal  raphe. 

cavernosa  is  foreshadowed  as  early  as  the  third  month  by  the 
appearance  in  the  penis  of  capillary  blood-vessels,  which,  in 
the  sixth  month,  undergo  marked  dilatation. 


THE  GLANDS  OF  COWPER.  239 

The  groove  on  the  under  surface  of  the  penis  becomes 
deeper,  and  the  genital  folds,  which  bound  the  groove  laterally, 
increase  in  size.  This  groove  extends  from  the  orifice  of  the 
urogenital  sinus  to  the  glans  penis.  The  genital  folds,  which 
in  the  female,  remain  distinct  and  become  the  nymphse,  unite 
with  each  other  in  the  male  and  convert  the  groove  into  a 
canal,  which  latter  is  practically  an  extension  of  the  uro- 
genital sinus  along  the  entire  length  of  the  penis  to  the 
glans.  The  canal  thus  formed  is  the  anterior  part  of  the 
male  urethra,  or,  in  other  words,  it  includes  all  of  the  urethra 
except  its  prostatic  portion,  which  represents  the  urogenital 
sinus.  The  orifice  of  this  newly-formed  canal,  situated  in 
the  glans,  is  the  meatus  urinarius.  Failure  of  union  of  the 
genital  folds,  either  wholly  or  in  part,  results  in  total  or  in 
partial  deficiency  of  the  floor  of  the  urethra,  this  anomaly 
being  known  as  hypospadias.  If  the  defective  closure  in- 
volves only  the  glans,  the  condition  is  denominated  glandular 
hypospadias. 

The  genital  folds  form  not  only  the  sides  and  the  floor  of 
the  penile  urethra,  but  by  an  extension  of  their  growth,  also 
its  roof,  thus  completely  surrounding  it.  Upon  the  acquisi- 
tion, by  the  now  united  genital  folds,  of  blood-vessels  and 
cavernous  spaces,  they  become  the  corpus  spongiosum  of  the 
penis,  and  thus  is  established  the  permanent  or  adult  relation 
of  these  parts. 

The  genital  ridge  becomes  diiferentiated  into  two  promi- 
nent folds  or  pouches  placed  one  on  either  side  of  the  root  of 
the  penis.  In  the  fourth  month  these  unite  to  form  the 
scrotum,  the  line  of  union  being  indicated  by  the  raphe. 
Failure  of  union  of  the  two  halves  of  the  scrotum  is  one  of 
the  features  of  certain  forms  of  so-called  hermaphroditism. 

The  glands  of  Cowper,  which  correspond  to  the  glands  of 
Bartholin  of  the  female,  are  developed,  like  the  latter,  as 
evaginations  of  the  terminal  part  of  the  urogenital  sinus. 

The  accompanying  tabulation  exhibits  a  comparison  of  the 
organs  of  the  two  sexes  on  the  basis  of  their  common  origin. 
Male  and  female  parts  that  develop  from  the  same  fetal 
structure  are  said  to  be  homologous  with  each  other. 


240 


TEXT-BOOK  OF  EMBRYOLOGY. 


Homologies  of  the  Sexual  System. 


Fetal  Structure. 

Indiflerent  sexual  gland. 
Wolffian  body — 

Its  middle  series  of  tu- 
bules and 

Corresponding  part  of 
Wolffian  duct. 

Remainder  of  Wolffian 
duct. 

IJpper    series    of    short 

tubules    (pronephros). 

Loiver  series   of  tubules. 

Duct  of  Miiller— 
Its  upper  extremity. 
Succeeding  portion. 


Female  Organs. 
Ovary. 

Short  tubules  of  par- 
ovarium and 

Horizontal  or  long  tube 
of  pai'ovarium. 

Usually  altogether  dis- 
appears ;  if  persistent, 
Gartner's  duct. 

Stalked  hydatid  of  Mor- 
gagni. 

Paroophoron. 


Fimbria  of  oviduct. 
Oviduct. 


Remaining    portion,   by 
fusion  with  its  fellow. 


Uterus  and  vagina. 


Male  Organs. 

Testis. 

Vasa  efferentia,  rete  testis 

and  coni  vasculosi. 
Tube  of  epididymis. 

Vas     deferens,     seminal 

vesicle,    and    ejacula- 

tory  duct. 
Stalked  hydatid  of  Mor- 

gagni. 
Paradidymis   (organ  of 

Giraldds). 

Sessile  hydatid. 
Usually    disappears ;    if 

persistent,      duct     of 

Rathke. 
Uterus  masculinus. 


Fetal  Structure. 
Genital  eminence. 
Genital  folds. 


Genital  ridge. 
Urogenital  sinus. 


External  Organs. 

Female  Organs. 
Clitoris. 

Nymphse  and  bulbi  ves- 
tibuli. 


Labia  majora. 
Urethra  and  vestibule. 
Glands  of  Bartholin. 


Male  Organs. 

Penis. 

Corpus  spongiosum,  en- 
closing spongy  part  of 
urethra. 

Scrotum. 

Prostatic  urethra,  mem- 
branous urethra,  pros- 
tate, Cowper"  s  glands. 


SUMMARY. 

1.  The  male  and  the  female  internal  generative  organs,  as 
well  as  the  kidney  and  the  ureter,  originate  from  the  meso- 
thelial  lining  of  the  body-cavity,  being  directly  produced 
from  the  Wolffian  body  and  the  duct  of  Miiller. 

2.  The  bladder  and  the  female  urethra,  but  in  the  male 
only  the  prostatic  urethra,  result  from  the  metamorphosis 
of  the  intra-embryonic  part  of  the  allantois,  and  are  therefore 
to  be  regarded  as  of  entodermic  origin. 

3.  Before  the  establishment  of  the  permanent  kidney,  a 


THE  MESONEPHROS.  241 

temporarily  functionating  organ,  the  mesonephros,  performs 
the  office  of  a  kidney  during  a  part  of  fetal  life,  and  this 
latter  is  preceded  by  the  pronephros,  an  organ  which,  though 
represented  in  the  higher  vertebrates  by  a  vestigial  remnant, 
is  functionally  active  only  in  larval  Amphibia  and  in  bony 
fishes. 

4.  The  Pronephros. — The  mesothelial  cells  of  the  outer  or 
parietal  wall  of  the  body-cavity  become  invaginated  in  a  line 
parallel  with  the  axis  of,  the  body,  and  the  cord  of  cells  thus 
formed  becomes  hollowed  out  to  constitute  the  pronephric  or 
segmental  duct.  At  several  points  this  duct  retains  its  con- 
nection with  the  surface-cells  by  means  of  cell-cords,  which 
latter  become  tubes  and  acquire  glomeruli.  The  long  tube 
and  the  shorter  tubules  with  their  glomeruli  constitute  the 
pronephros. 

5.  The  Mesonephros. — The  transverse  segmentation  of  the 
middle  plate,  which  connects  the  paraxial  mesoderm  with 
the  parietal  plate,  results  in  the  formation  of  a  series  of 
cell-masses,  the  nephrotomes.  Each  nephrotome  becomes 
a  tube  and  acquires  one  or  more  glomeruli.  The  deeper 
ends  of  the  tubes  become  connected  with  the  pronephric 
or  segmental  duct,  which  latter  is  known  henceforth  as 
the  mesonephric  or  Wolffian  duct.  These  tubes,  with  the 
adjacent  part  of  the  Wolffian  duct,  constitute  the  mesoneph- 
ros, which  functionates,  for  a  time,  even  in  the  human  fetus, 
as  an  organ  of  urinary  excretion.  The  entire  Wolffian  duct, 
with  the  pronephric  tubules  and  the  mesonephric  tubules^ 
constitutes  the  Wolffian  body.  The  AVolffian  duct  opens  at. 
its  lower  or  caudal  extremity  into  the  cloaca. 

6.  The  metanephros  or  permanent  kidney  develops  from 
a  small  diverticulum  that  pouches  out  from  the  lower  or 
caudal  end  of  the  Wolffian  duct.  The  uriniferous  tubules  and 
the  pelvis  of  the  kidney  correspond  to  the  dilated  and  sub- 
divided fundus  of  this  diverticulum,  while  the  ureter  repre- 
sents its  stalk.  The  surrounding  mesodermic  tissue  furnishes 
all  the  component  elements  of  the  ureter-walls  and  of  the 
kidney  except  the  epithelial  j^arts,  which  latter,  as  noted 
above,  proceed  from  the  Wolffian  body. 

16 


242  TEXT-BOOK   OF  EMBRYOLOGY. 

7.  The  suprarenal  bodies  probably  are  derived,  in  part, 
from  epithelial  outgrowths  which  proceed  from  the  meso- 
nephros  to  form  the  cortical  part  of  the  organ,  and,  in  part, 
from  chains  of  small  cells  that  bud  forth  from  the  embryonic 
sympathetic  ganglia  to  form  its  medulla.  The  surrounding 
mesodermic  tissue  contributes  the  connective-tissue  parts  of 
the  suprarenal  body. 

8.  The  sexual  apparatus  in  its  earlier  stages  presents  no 
distinctions  of  sex.  The  elements  of  this  early  indifferent 
type  are  the  indifferent  genital  gland,  the  Wolffian  duct,  and 
the  duct  of  Miiller, 

9.  The  indiflferent  genital  gland  originates  in  the  meso- 
thelium  of  the  body-cavity.  The  mesothelial  cells  overlying 
the  ventroraesial  aspect  of  the  Wolffian  body  undergo  mul- 
tiplication in  the  fifth  week  and  thereby  produce  an  elongated 
elevation,  the  genital  ridge.  Further  multiplication  of  its 
cells  and  the  addition  of  other  elements  bring  about  the 
transformation  of  this  ridge  into  the  well-defined  genital 
gland,  which  now  lies  in  close  relation  with  the  Wolffian 
tubules.  The  mesothelial  cells  are  the  "germinal  epithe- 
lium "  of  Waldeyer,  the  cells  that  produce  the  ova  or  the 
spermatozoa,  according  to  the  future  sex. 

10.  The  duct  of  Miiller  makes  its  appearance  soon  after 
the  Wolffian  duct.     It  lies  parallel  with  and  to  the  outer 

"side  of  the  Wolffian  duct  and  also  terminates  in  the  cloaca. 
It  is  of  mesodermic  origin,  being  produced  either  by  evagi- 
nation  of  the  mesothelial  cells  of  the  body-cavity,  or  by  a 
splitting  off  from  the  Wolffian  duct. 

11.  The  generative  systems  of  both  sexes  result  from  the 
metan:iorphosis  of  the  three  structures  making  up  the  early 
indifferent  sexual  apparatus — namely,  the  indifferent  sexual 
gland,  the  Wolffian  body,  and  the  duct  of  Miiller. 

12.  The  male  sexual  system  is  produced  by  the  transfor- 
mation of  the  indifferent  gland  into  the  testicle,  and  the  con- 
version of  the  Wolffian  tubules  and  the  Wolffian  duct  into 
the  system  of  excretory  ducts  for  that  gland,  the  short  tubules 
becoming  the  vasa  efferentia  and  coni  vasculosi,  while  the 
Wolffian  duct  itself  furnishes  the  body  and  the  globus  minor 


FEMALE  EXTERNAL   GENITALL4..  243 

of  the  epididymis,  the  vas  deferens,  the  vesicula  seminalis, 
and  the  ejaculatory  duct.  The  duct  of  Miiller  remains  un- 
developed and  is  represented  in  the  adult  by  the  atrophic 
sessile  hydatid  and  the  uterus  masculinus. 

13.  The  female  sexual  apparatus  is  brought  about  by  the 
development  of  the  indifferent  gland  into  the  ovary,  and  by 
the  metamorphosis  of  the  upper  segments  of  the  ducts  of 
Miiller  into  the  Fallopian  tubes,  and  the  fusion  of  the  re- 
maining portions  of  the  two  ducts  to  form  the  uterus  and 
the  vagina.  The  Wolffian  duct  and  tubules  give  rise  to 
atrophic  structures  in  the  female,  the  most  conspicuous  of 
which  is  the  parovarium  or  epoophoron. 

14.  Both  the  male  and  the  female  external  genitalia  are 
developed  from  fetal  structures  common  to  the  two  sexes, 
the  genital  eminence,  the  genital  ridge,  and  the  genital  folds. 
The  genital  eminence  is  situated  at  the  anterior  or  ventral 
part  of  the  cloacal  depression.  The  genital  ridge  is  an  eleva- 
tion surrounding  this  pit  and  the  genital  eminence,  while  the 
genital  folds  are  on  the  under  surface  of  the  genital  eminence, 
one  on  each  side  of  a  longitudinal  groove. 

15.  The  Wolffian  ducts  and  the  ducts  of  Miiller  open  into 
the  cloaca,  but  when  that  aperture  becomes  differentiated  into 
the  anus  and  the  urogenital  sinus,  as  it  does  at  the  fourteenth 
week,  these  ducts  fall  to  the  latter  apartment.  The  orifice 
of  the  urogenital  sinus  being  at  the  base  of  the  genital  emi- 
nence, the  sinus  comes  into  continuitv  with  the  groove  on  the 
under  surface  of  the  eminence. 

16.  The  female  external  genitalia  are  produced  by  the 
further  development  of  the  three  structures  mentioned  above. 
The  genital  eminence  becomes  the  clitoris.  The  genital  folds 
on  the  under  surface  of  the  clitoris  become  somewhat  pro- 
longed to  constitute  the  labia  minora.  The  genital  ridge 
becomes,  anteriorly,  the  mons  veneris  and  laterally  the  labia 
majora.  The  orifice  of  the  urogenital  sinus  is  represented  by 
the  vestibule,  and  since  the  Miillerian  ducts  near  their  ter- 
mination in  the  urogenital  sinus  fuse  to  form  the  vagina,  the 
latter  passage  opens  in  the  adult  into  the  vestibule.  Since, 
also,  the  urogenital  sinus  receives  the  termination  of  the 


244  TEXT-BOOK  OF  EMBRYOLOGY. 

allantois,  which  becomes  the  female  urethra,  the  latter  canal 
likewise  opens  into  the  adult  vestibule. 

17.  The  male  external  genitals  represent  a  further  de- 
velopment of  the  embryonic  genital  eminence,  genital  folds, 
and  genital  ridge  than  do  the  female  organs.  The  geni- 
tal eminence  becomes  the  penis,  the  genital  folds,  uniting 
with  each  other  so  as  to  surround  the  groove,  producing 
the  corpus  spongiosum.  The  groove  itself,  being  thus 
converted  into  a  canal  which  extends  the  now  closed  uro- 
genital sinus  to  the  end  of  the  penis,  constitutes  all  of 
the  male  urethra  except  the  first  or  prostatic  portion.  The 
prostatic  urethra  represents  the  proximal  extremity  of  the 
allantois.  Since  the  Wolffian  ducts  open  into  the  urogenital 
sinus  after  the  division  of  the  cloaca,  the  terminations  of  those 
ducts,  represented  now  by  the  ejaculatory  ducts,  open  into 
the  pi'ostatic  urethra ;  and  since  the  Miillerian  ducts  also 
open  into  the  urogenital  sinus,  the  uterus  masculinus,  which 
is  the  representative  in  the  male  of  the  terminal  parts  of  the 
Miillerian  ducts,  is  found  likewise  in  the  prostatic  urethra. 
The  lateral  parts  of  the  genital  ridge,  which,  in  the  female, 
become  the  labia  raajora,  fuse  with  each  other  in  the  male  to 
form  the  scrotum. 

18.  The  condition  of  so-called  hermaphroditism  may  be 
produced  either  by  an  unusual  degree  of  development  of  the 
female  external  genitals,  resulting  in  a  clitoris  resembling  a 
penis  and  in  labia  majora  which  simulate  a  cleft  scrotum  ;  or 
by  the  arrested  development  of  male  organs,  whereby  the 
genital  folds  and  the  genital  ridges  fail  to  unite,  the  urethra 
in  consequence  opening  at  the  base  of  the  penis. 


CHAPTER    XIY. 

THE   DEVELOPMENT  OF  THE   SKIN   AND   ITS 
APPENDAGES. 

The  appendages  of  the  skin  include  the  sebaceous  and 
sweat  glands,  the  mammary  glands,  the  nails,  and  the  hairs. 


THE   SKIN. 


The  skin,  consisting  of  the  epid 
true  skin,  or  derm,  or  corium,  is 
the  epithelial  epidermis  being  a 
product  of  the  ectoderm,  and  the 
corium  originating  from  the  meso- 
derm. The  nails  and  hairs  are 
outgrowths  of  the  epithelial 
layer,  while  the  various  glands 
are  derived  from  infoldings  or 
invaginations  of  the  same 
stratum. 

The  corium,  the  connective- 
tissue  component  of  the  skin,  is 
an  outgrowth  of  the  cutis  plates 
of  the  primitive  segments  or 
somites  (Fig.  117).  It  first  ap- 
pears in  crude  form  in  the 
second  month  as  a  layer  of 
spindle-cells  beneath  the  ecto- 
derm. In  the  third  month,  the 
more  superficial  part  of  this 
layer  acquires  more  definite  and 
distinctive  character,  the  rather 
loose  aggregation  of  cells  having 
differentiated  into  a  tissue  which 
is  a  mesh-work  of  bundles  of 


ermis  or  cuticle  and  of  the 
derived  from  two  sources. 


Fig.  117.— Cross-section  through 
the  region  of  the  pronephros  of  a 
selachian  embryo  in  which  the  myo- 
tomes are  in  process  of  being  con- 
stricted off  (Hertwig):  nr,  neural 
tube;  eh,  chorda;  ao,  aorta;  mp, 
muscle-plate;  cp,  cutis  plate;  vb, 
middle  plate ;  sk,  skeletogenous  tis- 
sue ;  vn,  pronephros ;  mk\  mk",  pari- 
etal and  visceral  mesoderm:  Ih, 
body-cavity;  ik,  intestinal  ecto- 
derm ;  h,  cavity  of  somite. 
245 


246  TEXT-BOOK  OF  EMBRYOLOGY. 

white  fibrous  connective  tissue  with  some  intermingled  elastic 
and  muscular  fibers  ;  this  constitutes  the  corium  proper.  The 
deeper  layer  of  cells  becomes  a  loose,  subcutaneous  areolar 
tissue  containing  a  few  scattered  fat-cells.  About  a  month 
later  the  external  surface  of  the  primitive  corium  loses  its 
smooth  character  and  presents  numerous  little  elevations,  the 
villi,  which  project  into  the  overlying  epidermis.  The  villi, 
being  highly  vascular,  play  an  important  part  in  the  nutrition 
of  the  epidermis  and  being  also  freely  supplied  with  nerves 
they  sustain  an  equally  important  relation  to  the  sensitiveness 
of  the  skin. 

From  the  middle  of  fetal  life  onward,  the  fat-cells  in  the 
subcutaneous  tissue  increase  in  number  to  such  extent  that 
there  is  formed  a  continuous  and  well-marked  subcutaneous 
layer  of  fat,  the  panniculus  adiposus. 

Certain  of  the  cells  of  the  primitive  corium  differentiate 
into  unstriated  muscular  tissue,  forming  thus  the  muscles 
of  the  hair-follicles,  the  arrectores  piloriun,  as  well  as  the 
subcutaneous  muscular  tissue  of  the  dartos  of  the  scrotum 
and  penis,  and  that  of  the  nipple  and  of  the  perineum. 

The  epidermis,  consisting  of  the  superficial  horny  layer 
and  the  deeper  mucous  layer  or  stratum  Malpighii,  is  entirely 
an  epithelial  structure.  Its  elements  are  simply  the  descend- 
ants of  the  early  ectodermic  cells  specially  modified  to  afford 
the  necessary  protection  to  the  more  sensitive  and  delicate 
corium. 

The  division  into  the  two  strata  of  the  epidermis  is  indi- 
cated a"?  early  as  the  latter  part  of  the  first  month,  at  which 
time  the  cells  of  the  ectoderm  have  become  arranged  into 
two  single  layers,  a  superficial  layer  of  rather  large  flattened 
cells  and  an  underlying  stratum  of  smaller  elements.  The 
cells  of  the  outer  layer,  or  epitrichium,  which  probably  rep- 
resents tlie  future  stratum  corneum,  successively  undergo 
degeneration  and  desquamation,  the  places  of  those  lost 
being  supplied  by  the  formation  of  new  ones  from  the  deeper 
layer.  As  time  goes  on,  both  layers  increase  in  thickness 
and  the  hairs  and  the  glands  of  the  skin  are  gradually 
formed.     With  increased  proliferation  there  is  increasingly 


DEVELOPMENT  OF  APPENDAGES  OF  THE  SKIN.   247 

active  desquamation  of  superficial  cells,  and  as  the  degenerate 
and  cast-oif  cells  become  mixed  with  the  products  of  the 
sebaceous  glands,  there  is  formed  a  sort  of  cheesy  coating 
of  the  skin,  the  vernix  caseosa  or  smegma  embryonum.  This 
is  first  easily  recognizable  in  the  sixth  month,  and  first  covers 
the  entire  surface  of  the  body  in  the  eighth  month.  It  serves 
to  protect  the  epidermis  of  the  fetus  from  maceration  in  the 
amniotic  fluid. 

The  completion  of  the  epidermis,  aside  from  the  develop- 
ment of  its  accessory  parts,  consists  simply  in  further  increase 
in  thickness  and  in  the  modification  of  the  superficial  cells 
to  produce  the  characteristic  scale-like  elements  of  the  cor- 
neous layer  of  the  skin,  accompanied  by  the  diiferentiation_ 
of  the  deeper  cells  into  those  of  the  rate  mucosum  or  stratum 
Malpighii.  The  extent  to  which  these  modifications  are  car- 
ried varies  in  diiferent  regions  of  the  body. 

THE  DEVELOPMENT  OF  THE  APPENDAGES  OF  THE  SKIN. 

The  Nails. — The  nails  have  their  beginning  in  little  claw- 
like  projections,  the  primitive  nails,  that  appear  upon  the 
tips  of  the  still  imperfect  fingers  and  toes  in  the  seventh 
week.^  These  result  from  localized  proliferation  of  the  cells 
of  the  epidermis,  being  entirely  epithelial  structures.  The 
rudimentary  nails  project  from  the  tips  of  the  digits,  instead 
of  occupying  the  dorsal  position  of  the  completed  structures. 

The  claw-like  primitive  nail,  between  the  ninth  and  twelfth 
"weeks,  becomes  surrounded  by  a  groove,  which  serves  to  sepa- 
rate it  from  the  general  ectoderraic 
surface.  These  claw-like  rudiments 
of  the  human  nails  are  quite  similar 
to  the  primitive  claws  of  many  mam- 
mals, the  primitive  nail  in  each  case         Fig.  lis.— Longitudinal  sec- 

1    J-  It  J.     j_i  -1  tion  through  the  toe  of  a  Cer- 

mcludmg  a    dorsal    part,    the    nail-  copithecus=>  (after  Oegenbaur): 

plate,  and  a   portion    whicll    belongs  «P,  nail-plate ;  s/i,  plantar  hom 

,  .  „  „     ,         ,,    .  (Sohlenhorn) ;  ww,  nail-waU. 

to  the  ventral  surface  oi  the  digit, 

called  the  plantar  horn  (Fig.  118).     The  striking  difference 

between  the   nails  of  the  human  adult  and  the  claws  and 

^  Or  ninth  week,  Minot.  ^  A  genus  of  long-tailed  African  monkeys.    ■ 


248  TEXT-BOOK  OF  EMBRYOLOGY. 

hoofs  of  many  animals  is  due  in  great  measure  to  the 
degree  of  development  t<j  which  this  ventrally  situated 
plantar  horn  attains.  In  the  hoofed  mammals  (Ungulata) 
and  the  clawed  mammals  (Unguiculata),  the  plantar  horn 
undergoes  very  great  development,  whereas  in  man  it  retro- 
grades and  leaves  no  trace  except  the  nail-welt,  or  the  narrow 
line  of  thickened  epidermis  where  the  distal  end  of  the  nail- 
lied  merges  into  the  ordinary  skin.  After  the  atrophy  of  the 
plantar  horn,  the  dorsally  situated  nail-plate  being  alone 
present,  the  rudimentary  nail  bears  a  greater  resemblance 
to  the  adult  condition. 

As  the  nail-plate  gradually  acquires  more  distinctive  char- 
acter, the  deeper  layers  of  the  skin  specialize  into  a  structure 
adapted  to  its  nutrition.  This  is  the  nail-bed,  a  highly  vas- 
cular and  sensitive  cushion  consisting  of  the  corium  and  of 
the  stratum  Malpighii  of  the  epidermis.  It  is  especially 
from  the  proximal  part  of  the  nail-bed,  representing  the 
matrix  of  the  fully-formed  condition,  that  the  nail  grows. 
The  rate  of  growth  is  such  that  the  ends  of  the  nails  pro- 
trude beyond  the  tips  of  the  digits  in  the  eighth  month. 

The  tissue  of  the  fully-formed  nail  corresponds  to  the 
stratum  lucidum  of  the  typical  epidermis,  developed  to  an 
unusual  degree.  The  epitrichium  or  future  stratum  comeum, 
the  most  superficial  layer  of  the  epidermis,  does  not  form  a 
part  of  the  nail,  but  constitutes  a  thin  covering,  the  epony- 
chium ;  this  is  lost  in  the  seventh  month,  with  the  exception 
of  a  small  band  over  the  root  of  the  nail,  which  persists  for 
a  short  time  as  the  perionyx. 

The  nails  of  the  toes  are  always  somewhat  behind  those  of 
the  fingers  in  development. 

To  repeat,  the  claw-like  rudimentary  nails  appear  in  the 
seventh  week,  the  nails  are  perfectly  formed  about  the  twelfth 
week,  and  break  through  their  epidermal  covering  in  the 
seventh  month,  reaching  to  or  beyond  the  finger-tips  in  the 
eighth  month. 

The  Hair. — Each  hair  consists  of  the  projecting  shaft  and 
the  embedded  root,  with  its  expanded  deep  extremity,  the 
hair-bulb,  the  root  being  embraced  by  the  hair-follicle.     The 


THE  ROOT  AND   THE  SHAFT. 


249 


liair  is  entirely  of  ectodermic  origin,  being  derived  from  the 
epidermal  layer  of  the  skin,  while  the  hair-follicle  is  partly 
derived  from  the  epidermis  and  in  part  is  a  product  of  the 
corium.  The  hairs  are  homologous  with  the  feathers  and 
scales  of  the  lower  animals. 

The  development  of  the  hair  is  initiated  in  the  third  fetal 
month  by  the  appearance  of  small  solid  masses  of  epithelium 
in  the  stratum  Malpighii  of  the  epidermis.  The  epithelial 
plugs  or  hair-germs  grow  into  the  underlying  corium  and 
are  met  by  outgrowths  or  papillse  of  the  latter,  which  develop 
almost  simultaneously.  The  papillse  are  very  vascular  and 
serve  for  the  nutrition  of  the  developing  hair. 

The  root  and  the  shaft  of  the  rudimentary  hair  result  from 
the  specialization  of  the  axial  or  central  cells  of  the  hair- 
germ.  These  cells  lengthen  in  the  direction  of  the  long  axis 
of  the  hair-germ  and  become  hard  and  corneous,  thus  con- 
stituting the  root  and  the  shaft,  the  cells  of  the  deepest  part 
of  the  hair-germ  forming  the  bulb.     The  growth  of  the  hair 


Fig.  119.— Two  diagrams  of  the  development  of  the  hair  (Hertwig) :  A  and  B, 
two  different  stages  of  the  development  of  the  hair  in  human  embryos  ;  ho,  cor- 
neous layer  of  the  epidermis;  scW,  mucous  layer;  pa,  hair-papilla ;  hk,  germ  of 
hair  ;  hs,  bulb  of  hair ;  ha,  young  hair  ;  aw,  iw,  outer  and  inner  sheaths  of  the  root 
of  the  hair  ;  hb,  hair-follicle  ;  td,  sebaceous  gland. 

in  length  is  due  to  the  proliferation  and  specialization  of  the 
cells  of  the  bulb.  The  papilla  of  the  underlying  corium 
indents  the  deep  surface  of  the  hair-bulb,  this  close  relation 
of  the  two  structures  enabling  the  papilla  the  better  to  fulfil 
its  function  of  providing  nourishment  to  the  bulb. 


250  TEXT-BOOK  OF  EMBRYOLOGY. 

The  hair-follicle,  consisting  of  an  outer  connective-tissue 
portion  or  fibrous  layer  and  an  inner  epithelial  part,  the 
inner  and  outer  root-sheaths,  is  partly  of  niesodermic  and 
partly  of  ectodermic  origin.  The  inner  and  outer  root- 
sheaths  are  produced  by  the  peripheral  cells  of  the  hair- 
germ  augmented  by  cells  contributed  directly  by  the  stratum 
Malpighii  of  the  epidermis.  The  outer  fibrous  constituent 
of  the  follicle  results  from  the  niesodermic  cells  of  the  corium 
that  immediately  surround  the  hair-germ. 

Gradually  increasing  in  length  by  the  addition  of  new 
cells  from  the  hair-bulb,  the  primitive  hair  at  length  pro- 
trudes from  the  follicle  as  free  hair.  This  first  growth  of 
hair  is  unpigmented  and  is  extremely  fine  and  soft,  being 
known  as  the  lanugo  or  embryonal  down.  This  appears  upon 
the  scalp  and  some  other  parts  of  the  body  in  the  fourth 
month,  gradually  extending  over  the  entire  surface  in  the 
succeeding  months.  In  the  eighth  month  the  lanugo  begins 
to  disappear,  but  is  not  lost  as  a  whole  until  after  birth,  when 
the  permanent  growth  of  hair  takes  its  place.  Upon  the  face, 
in  fact,  the  lanugo  persists  throughout  life. 

The  development  of  the  secondary  hair  is  still  a  disputed 
point.  It  is  claimed  by  some  authorities  (Stieda,  Feiertag) 
that  they  develop  from  entirely  new  hair-germs,  while  others 
{e.g.,  Hertwig)  hold  that  the  bulb  for  the  new  hair  buds  off 
from  the  atrophic  bulb  of  the  hair  just  lost. 

The  Sebaceous  and  Sweat-glands. — The  sweat-glands,  in- 
cluding not  only  the  sweat-glands  proper  but  the  ceruminous 
glands  of  the  external  auditory  meatus  and  the  glands  of  Moll 
of  the  eyelids,  are  derived  from  the  ectodermic  epithelium. 
The  glands  are  of  the  simple  tubular  type.  Each  gland 
develops  from  a  small  accumulation  of  epidermal  cells  that 
grows,  in  the  fifth  month,  from  the  Malpighian  or  mucous 
layer  of  the  epidermis  into  the  underlying  corium.  The 
solid  epithelial  plugs  become  tubes  in  the  seventh  month  by 
the  degeneration  and  final  disappearance  of  the  central  cells. 
The  deeper  part  of  the  tube  becomes  coiled  and  its  lining 
epithelium  takes  on  the  characteristics  of  secreting  cells. 
Some  of  the  cells  of  the  original  epithelial  plug  undergo 


THE  MAMMARY  GLAND.  251 

specialization  into  muscular  tissue,  thus  producing  the  involun- 
tary muscles  of  the  sweat-glands. 

The  sebaceous  glands  are  developed  from  solid  epithelial 
processes  that  originate  from  the  deep  layer  or  rete  mucosuni 
of  the  epidermis  in  a  manner  similar  to  that  of  the  develop- 
ment of  the  sweat-glands.  There  is  the  difference,  however, 
that  the  epithelial  plugs  acquire  lateral  branches  and  thus 
usually  produce  glands  of  the  compound  saccular  or  acinous 
variety.  There  is  the  further  difference  that  the  epithelial 
outgrowths  generally  develop  from  the  ectodermic  cells  of 
the  outer  sheath  of  the  root  of  the  hair  near  the  orifice  of 
the  follicle  (Fig.  119,  id),  in  consequence  of  which  the  ducts 
of  the  finished  glands  usually  open  into  the  hair-follicles.  In 
some  regions,  however — regions  devoid  of  hair,  as  the  prepuce 
and  the  glans  penis,  the  labia  minora,  and  the  lips — the 
growth  is  directly  from  the  stratum  Malpighii,  as  in  the  case 
of  the  sweat-glands. 

The  Mammary  Gland. — The  mammary  gland  represents  a 
number  of  highly  specialized  sebaceous  glands  so  associated 
as  to  constitute  the  single  adult  structure.  Its  origin,  there- 
fore, is  to  be  sought  in  the  cells  of  the  epidermis  in  common 
with  that  of  the  ordinary  sebaceous  glands. 

The  development  of  the  milk-glands  is  begun  as  early  as 
the  second  month.  At  this  time  the  deep  layer  of  the 
epidermis,  in  the  sites  of  the  future  glands,  becomes  thick- 
ened by  the  multiplication  of  its  cells,  the  thickened  patch 
encroaching  upon  the  underlying  corium  (Fig.  120,  A,  b). 
This  thickened  area  enlarges  somewhat  peripherally  and  its 
margins  become  elevated,  owing  to  which  latter  circumstance 
the  patch  appears  relatively  depressed  [B).  The  depression  is 
known  as  the  glandular  area,  and  it  corresponds  with  the 
future  areola  and  nipple. 

From  the  bottom  of  the  glandular  area,  numerous  small 
masses  or  bud-like  processes  of  cells  grow  down  into  the 
corium.  Some  of  the  buds  acquire  lateral  branches.  By 
the  hollowing  out  of  these  cell-buds  the  latter  are  transformed 
into  tubes  (c),  which  open  upon  the  glandular  area.  The  branch- 
ing of  the  cords  begins  in  the  seventh  month  and  is  carried 


252 


TEXT-BOOK  OF  EMBRYOLOGY. 


on  to  such  a  degree  that  each  original  cell-cord  gives  rise 
to  a  tubo-racemose  gland.  Tiie  hollowing  out  of  the  solid 
processes  begins  shortly  before  birth,  but  is  not  completed 
until  after  that  event.  Each  cell-cord  becomes,  in  the  strict 
sense,  a  complete  gland,  each  such  individual  structure  form- 
ing a  lobe  of  the  mature  organ. 

This  stage  of  the  human  mammary  gland — that  is,  a  de- 
pressed gland-area  upon  which  open  individual  glands,  the 


Fig.  120.— Sections  representing  tliree  successive  stages  of  development  of  the 
human  mamma  (Tourneux) :  A,  fetus  of  32.40  mm.  (1.3  in  ) ;  B,  of  10.16  cm.  (4  in.); 
C,  of  24.35  cm.  (9.6  in.);  a,  epidermis;  6,  aggregation  of  epidermal  cells  forming 
anlage  of  gland ;  c,  galactophorous  ducts ;  d,  groove  limiting  glandular  area ;  e, 
great  pectoral  muscle ;  /,  unstriated  muscular  tissue  of  areola ;  g,  subcutaneous 
adipose  tissue. 


nipple  being  absent — is  the  permanent  condition  in  some  of 
the  lowest  mammals,  as  in  the  echidna,  one  of  the  mono- 
tremes.  In  all  higher  mammals,  however,  further  meta- 
morphoses occur  in  the  tissues  of  the  glandular  area,  and  in 
the  human  fetus  these  tissues  become  the  nipple  and  the  sur- 
rounding areola. 

The  nipple  is  partly  formed  before  birth,  but  does  not 
become  protuberant  until  post-fetal  life.  The  depressed 
glandular  area  rises  to  the  level  of  the  surrounding  parts, 
and  its  central  region,  which  includes  the  orifices  of  the 
already  formed  or  just  forming  ducts,  swells  out  into  a  little 
prominence,  the  nipple.  This  prominence  is  a  protrusion  of 
the  epidermis  and  includes  the  terminal  extremities  of  the 


AT  BIRTH.  253 

milk-ducts  as  well  as  the  blood-vessels  and  connective-tissue 
elements  which  surround  the  ducts.  In  the  dermal  con- 
stituent of  the  rudimentary  nipple  unstriated  muscular  tissue 
develops.  The  region  of  the  glandular  area  not  concerned 
in  the  formation  of  the  nipple  becomes  the  areola. 

At  birth,  as  above  intimated,  the  mammary  gland  is  still 
rudimentary,  since  many  of  the  ducts  have  not  yet  acquired 
their  lumina  nor  their  full  degree  of  complexity.  Shortly 
after  birth  a  small  quantity  of  milky  secretion,  the  so-called 
witches'  milk,  may  be  expressed  from  the  glands — in  the  male 
and  female  infant  alike.  This  is  true  milk  according  to 
Rein  and  Barfruth,  but  according  to  Kolliker,  it  is  merely  a 
milky  fluid  containing  the  debris  of  the  degenerated  central 
cells  of  those  rudimentary  ducts  that  were  still  solid  at  birth. 

So  far,  the  milk-glands  are  alike  in  the  two  sexes,  but 
while  in  the  male  they  remain  rudimentary  structures,  they 
continue  to  increase  both  in  size  and  in  complexity  in  the 
female.  The  increase  affects  not  only  the  glandular  tissue 
proper  but  the  connective-tissue  stroma  as  well.  At  the  time 
of  puberty  the  growth  of  the  glands  receives  a  new  impetus, 
which  is  very  materially  augmented  upon  the  occurrence  of 
pregnancy.  There  may  be  said,  therefore,  to  be  several  dis- 
tinct phases  in  the  development  of  the  milk-glands,  first,  the 
embryonic  stage ;  second,  the  infantile  stage ;  third,  the  stage 
of  maturity  beginning  at  the  time  of  puberty;  and  finally, 
the  stage  of  full  functional  maturity  consequent  upon  preg- 
nancy and  parturition. 


CHAPTER    XV. 
THE   DEVELOPMENT  OF  THE  NERVOUS  SYSTEM. 

The  nervous  system  of  the  adult,  including  the  cerebro- 
spinal axis  and  nerves,  and  the  sympathetic  system  of  ganglia 
and  nerves,  is  made  up  of  the  essential  neural  elements,  the 
neurons,  together  with  the  supporting  framework  or  stroma.^ 

The  neurons  and  a  part  of  the  stroma  result  from  the 
specialization  of  the  ectodermic  layer  of  the  embryo.  The 
ectodermic  origin  of  the  nervous  system  acquires  certain 
interest  in  view  of  the  conditions  that  obtain  in  some  of  the 
lowest  and  simplest  organisms.  For  example,  in  the  ameba, 
the  single  protoplasmic  cell  which  constitutes  the  entire  indi- 
vidual possesses  the  several  fundamental  vital  properties  of 
protoplasm,  such  as  respiration,  metabolism,  contractility, 
motility,  etc.,  in  equal  degree,  no  single  property  being  more 
highly  developed  than  the  others,  and  no  particular  part  of 
the  cell  exhibiting  greater  specialization  than  the  other  parts. 
In  other  words,  the  protoplasmic  substance  of  the  animal  is 
at  once  a  respiratory  mechanism,  a  nervous  apparatus,  and 
an  organ  for  the  execution  of  the  various  other  vital  func- 
tions. 

In  somewhat  more  highly  developed  creatures,  as  the 
infusoria,  although  there  is  no  differentiation  into  separate 
tissues  and  probably  not  even  into  separate  cells,  there  is  seen 
some  progress  toward  the  specialization  of  certain  parts  of 
the  organism  for  the  performance  respectively  of  the  different 
functions  of  life.     For  example,  the  central  part  of  the  ani- 

'  The  neurons  are  the  units  of  which  the  nervous  system  is  made  up. 
Each  neuron  consists  of  a  nerve-cell  with  everything  belonging  to  it — that 
is,  with  its  various  processes,   including  the  axis-cylinder  process  or  neurit, 
which  becomes  the  axis-cylinder  of  a  nerve-fiber. 
254 


THE  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM.   255 

mal  has  digestive  functions,  while  it  is  by  the  superficial 
portion  alone  that  the  creature  is  brought  into  relation  with 
the  outside  world,  the  sensitiveness  or  irritability  of  the 
surface,  by  which  the  animal  is  made  responsive  to  external 
impressions,  being  the  nearest  approach  to  the  function  of  a 
nervous  system  that  it  possesses. 

This  primitive  function  of  the  surface  of  the  organism 
is  suggestive  as  to  the  origin  of  the  nervous  system  of 
higher  type  creatures.  It  will  be  seen,  indeed,  that  not  only 
is  the  nervous  system  proper  derived  from  the  ectodermic 
cells  of  the  embryo  but  that  the  peripheral  parts  of  the 
organs  of  special  sense,  as  the  olfactory  epithelium,  the  organ 
of  Corti,  and  the  retina,  have  the  same  origin. 

The  alteration  of  those  cells  of  the  ectodermic  stratum 
that  are  to  specialize  into  nervous  elements  begins  prior  to 
the  fourteenth  day  in  the  human  embryo,  in  the  stage  of 
the  blastodermic  vesicle.  The  change  consists  in  a  gradual 
modification  of  the  form  of  the  cells,  the  cells  common 
to  the  general  surface  of  the  germ  assuming  the  col- 
umnar type.  The  process  affects  the  cells  of  the  median 
line  of  the  embryonic  area  in  advance  of  the  primitive  streak, 
resulting  in  the  production  of  a  thickened  longitudinal  median 
zone.  This  thickened  area  is  the  medullary  plate  (Fig.  32, 
p.  62).  On  each  side  of  the  plate — which  is  apparent  at  the 
fourteenth  day — the  adjoining  ectodermic  cells  become  heaped 
up  to  form  the  medullary  folds,  which  latter  therefore  bound 
the  medullary  plate  laterally.  The  medullary  plate  becomes 
concave  on  the  surface,  forming  the  medullary  groove  (Fig. 
121).  By  the  deepening  of  the  groove,  the  lateral  edges  of 
the  plate  approach  each  other  (Fig.  1 22),  and  finally  they  meet 
and  unite,  thus  producing  a  tube,  the  neural  tube  or  canal. 

Since  the  medullary  folds  similarly  meet  and  unite  with 
each  other — their  union  slightly  preceding  that  of  the  edges 
of  the  plate — the  neural  tube  comes  to  lie  entirely  beneath 
the  surface-ectoderm  and  soon  loses  all  connection  with  it. 
The  closing  of  the  tube  and  the  union  of  the  medullary  folds 
occur  first  near  the  anterior  end  of  the  embryonic  area,  in  a 
position  that  corresponds  with  the  region  of  the  future  neck, 


256 


TEXT-BOOK  OF  EMBRYOLOGY. 


and  from  this  point  it  proceeds  both  cephalad  and  caudad. 
Since  the  medullary  folds  at  their  caudal  extremity  embrace 


Medullary 
furrow. 


Amnion. 


Visceral 
mesoderm. 


Noioekord.  Somite.     Gut  entoderm. 

Fig.  121. — Transverse  section  of  a  sixteen-and-a-half-day  sheep  embryo  possessing 
six  somites  (Bonnet). 

the  primitive  streak  (Fig.  32,  p.  62),  the  latter  structure  is 
included  within  the  caudal  end  of  the  neural  tube  by  the 


Ectoderm. 


Amnion. 


Parietal 
mesodernt. 


Cell-mass/or  _ 
Wolffian  body. 


Primitive 
endothelium. 


Notochord. 
Fig.  122.— Transverse  section  of  a  fifteen-and-a-half-day  sheep  embryo  possessing 
seven  somites  (Bonnet). 

coming  together  of  the  folds,  and  thus  the  blastopore,  which 
was  previously  the  external   aperture  of  the   archenteron, 


THE  DEVELOPMENT  OF  THE  SPINAL  CORD.      257 

comes  to  constitute  the  neurenteric  canal,  or  an  avenue  of 
communication  between  the  neural  canal  and  the  primitive 
intestine. 

The  neural  canal  then  is  a  tube  composed  of  columnar 
cells,  which  is  formed  by  the  folding  in  of  the  ectoderm  and 
which  occupies  the  median  longitudinal  axis  of  the  embryonic 
area  and  consequently  of  the  future  embryonic  body.  •  Frora 
this  simple  epithelial  canal  the  entire  adult  nervous  system  is 
evolved. 

The  evolution  of  the  highly  complex  cerebrospinal  axis 
from  such  a  simple  structure  as  the  neural  canal  is  referable 
both  to  the  principle  of  unequal  growth — the  walls  of  the 
tube  becoming  thickened  by  the  multiplication  of  the  cells — 
and  to  the  formation  of  folds. 

The  portion  of  the  neural  canal — approximately  one-half — 
that  is  devoted  to  the  formation  of  the  brain  is  delimited 
from  the  part  that  produces  the  spinal  cord  by  the  dilatation 
of  the  anterior  or  liead-end  of  the  tube,  and  the  subsequent 
division  of  this  dilated  sac-like  portion  into  three  communi- 
cating sacs  called  respectively  the  fore-brain,  mid-brain,  and 
hind-brain  vesicles  (Fig.  126).  These  three  vesicles  give 
rise  to  the  brain,  while  the  remaining  part  of  the  neural  canal 
forms  the  spinal  cord. 

THE  DEVELOPMENT  OF  THE  SPINAL  CORD. 

In  the  growth  of  the  spinal  cord  from  the  spinal  portion 
of  the  neural  canal  we  have  to  consider  the  evolution  of  a 
cylindrical  mass  of  nerve-cells  and  nerve-fibers  with  the 
supporting  stroma  from  a  simple  epithelial  tube. 

The  wall  of  the  neural  tube,  although  consisting  at  first 
of  a  single  layer  of  epithelial  cells,  is  not  of  uniform  thick- 
ness throughout  its  circumference.  While  the  external  out- 
line is  oval,  the  lumen  of  the  tube  is  a  narrow  dorsoventral 
fissure  (Fig.  36,  p.  65).  The  cavity  is  therefore  bounded  on 
the  sides  by  thickened  lateral  columns,  while  the  dorsal  and 
ventral  walls,  which  connect  the  lateral  columns  with  each 
other,  are  thinner  and  are  called  respectively  the  roof-plate 
and  the  floor-plate. 
17 


258 


TEXT-BOOK   OF  EMBRYOLOGY. 


After  a  short  time,  the  walls  of  the  tube  having  thickened 
by  the  multiplication  of  the  cells,  the  shape  of  the  lumen 

alters,  two  laterally  projecting 
angles  being  added  (Fig.  123j. 
The  effect  of  this  change  is  to 
partially  divide  each  lateral  half 
into  a  dorsal  and  a  ventral 
region.  The  neural  canal  at 
this  stage  may  be  said  to  con- 
sist of  six  columns  of  cells,  the 
two  dorsal  zones  connected  with 
each  other  by  the  roof-plate,  and 
the  two  ventral  zones  united  by 
the  floor-plate.  These  regions 
are  also  distinguishable,  with 
certain  characteristic  modifica- 
tions, in  the  head-region  of  the 
tube.  They  are  important  in 
their  bearing  upon  the  further 
development  of  the  structure, 
since  the  dorsal  and  ventral 
zones  are  related  respectively  to  the  dorsal  or  sensory  and 
the  ventral  or  motor  roots  of  the  spinal  nerves. 

The  differentiation  of  the  cells  of  the  neural  tube  into  two 
kinds  of  elements,  one  of  which  gives  rise  to  sustentative 
tissue  or  neuroglia  while  the  other  produces  the  nerve-cells,  is 
observed  at  about  the  end  of  the  third  week.  The  single 
layer  of  "columnar  cells  which  at  first  composes  the  wall  of 
the  tube,  the  long  axes  of  the  cells  being  radially  arranged, 
soon  exhibits  near  the  lumen  a  row  of  round  cells,  probably 
the  first  offspring  of  the  columnar  cells.  The  round  cells 
are  the  germ-cells  or  germinating  cells,  from  which  are  devel- 
oped the  neuroblasts  or  young  nerve-cells.  All  the  other 
cells,  known  as  the  spongioblasts,  are  concerned  in  producing 
sustentative  tissue. 

The  stroma  of  the  central  nervous  system  includes  two 
constituents — a  connective-tissue  element,  and  a  part,  the 
neuroglia,  which  is  of  epithelial  origin,  and  which  is  not  to 


Fig.  123.— Transverse  section  of 
the  cervical  part  of  the  spinal  cord 
of  a  human  embryo  of  six  weeks,  X 
36  (from  KoUiker) :  c,  central  canal ; 
<?,  its  epithelial  lining;  ate'  (superi- 
orly), the  original  place  of  closure 
of  the  canal ;  a,  the  white  substance 
of  the  anterior  columns ;  g,  gray  sub- 
stance of  anterolateral  horn  ;  p,  pos- 
terior column  ;  ar,  anterior  roots ; 
pr,  posterior  roots. 


THE  DEVELOPMENT  OF  THE  SPINAL   CORD.        259 

be  regarded,  therefore,  as  connective  tissue.  The  connective- 
tissue  portion  of  the  stroma  is  produced  by  the  ingrowth  of 
the  pial  processes  from  the  pia  mater,  and  is  hence  of  meso- 
dermic  origin. 

The  neuroglia  is  derived  from  the  spongioblasts,  which 
result  from  the  specialization  of  the  large  columnar  cells  of 
which  the  wall  of  the  neural  canal  is  composed.  These  cells, 
whose  length  comprises  the  entire  thickness  of  the  wall  of 
the  tube  in  the  earliest  stages,  undergo  partial  absorption  and 
disintegration,  each  cell  being  transformed  into  an  elongated 
system  of  slender  processes  or  trabeculse,  and  each  such  system 
being  a  completed  spongioblast  (Fig.  124).    The  inner  ends  of 


Fig.  124. — Cross-section  through  the  spinal  cord  of  a  vertebrate  embryo  (after 
His) :  a,  outer  limiting  membrane ;  b,  outer  neuroglia  layer,  region  of  future  white 
matter ;  c,  germ-cells ;  d,  central  canal ;  e,  inner  limiting  membrane  or  ependymal 
layer;  /,  spongioblasts ;  g,  neuroblasts  (mantle  layer) ;  h,  anterior  root-fibers. 


the  spongioblasts  coalesce  with  each  other,  forming  thus  the 
internal  limiting  membrane,  while  the  peripheral  extremities 
interlace  with  each  other  to  form  a  close  network.  As  the 
walls  of  the  neural  tube  increase  in  thickness,  the  spongio- 
blasts become  more  and  more  broken  up  to  form  the  delicate 


260  TEXT-BOOK  OF  EMBRYOLOGY. 

neurogliar  network  with  interspersed  nucleated  glia  cells. 
Such  of  the  spongioblasts  as  border  the  cavity  of  the  neural 
tube  become  the  cells  of  the  later  ependyma  of  the  central 
canal  of  the  spinal  cord  and  of  the  ventricles  of  the  brain. 
The  cells  of  the  ependyma  become  ciliated  in  the  human 
fetus  in  the  fifth  week. 

The  nerve-cells  of  the  spinal  cord — as  also  of  the  brain — 
are  the  specialized  descendants  of  the  germ-cells  referred  to 
above.  The  proliferation  of  the  germ-cells  produces  the 
neuroblasts,  or  young  nerve-cells  (Fig.  124).  The  latter  ele- 
ments move  aM'ay  from  the  primitive  position  of  the  germ- 
cells  near  the  lumen  of  the  tube  and  develop  into  the  nerve- 
cells.  The  transition  is  effected  by  the  accumulation  of  the 
cell's  protoplasm  on  the  distal  side  of  the  nucleus  and  its 
elongation  into  a  process.  This  process  is  a  neurit  or  axis- 
cylinder  process  and  is  the  beginning  of  a  nerve-fiber.  The 
dendrites  or  protoplasmic  processes  appear  considerably  later. 
Some  of  the  fibers  thus  produced  grow  out  from  the  neural 
tube  to  constitute  the  efferent  fibers  of  the  peripheral  nerves^ 
while  others  contribute  to  the  formation  of  the  fiber-tracts 
of  the  cord. 

After  the  appearance  of  the  neuroblasts  and  developing 
nerve-cells,  the  wall  of  the  neural  tube  is  divisible  into  three 
layers  (Fig.  124);  an  inner  or  ependymal  layer,  next  the 
lumen  of  the  tube ;  adjoining  this,  the  mantle  layer,  made 
up  of  neuroblasts ;  and  a  peripherally  situated  neuroglia  layer, 
which  occupies  the  position  of  the  future  tracts  of  white 
fibers  of  the  cord. 

The  alterations  in  the  form  and  size  of  the  spinal  cord  go 
hand  in  hand  with  the  histological  changes  noted  above. 
While  those  areas  that  have  been  mentioned  as  the  dorsal  and 
ventral  zones  increase  greatly  in  thickness,  the  floor-plate  and 
the  roof-plate — the  ventral  and  dorsal  walls  of  the  neural 
tube — remain  thin  (Fig.  125).  They  are  never  invaded  by 
the  nerve-cells  but  consist  of  thin  layers  of  neuroglia  which 
later  become  penetrated  by  nerve-fibers  that  grow  from  one 
side  to  the  other.  They  thus  represent  the  anterior  and  pos- 
terior white  commissures  of  the  cord.     These  plates  remain 


THE  DEVELOPMENT  OF  THE  SPINAL   CORD.       261 

relatively  fixed  in  position  because  of  their  failure  to  expand, 
while  the  lateral  walls  of  the  tube  undergo  great  expansion, 
in  both  the  ventral  and  dorsal  directions,  as  well  as  laterally. 
In  this  way  a  median  longitudinal  cleft  is  produced  on  the 
ventral  wall  of  the  spinal  cord  and  a  similar  one  on  the 
dorsal  wall.  These  are  the  anterior  and  posterior  median 
fissures.  Since  the  so-called  posterior  median  fissure  is  not  a 
true  fissure  but  merely  a  septum,  it  differs  from  the  anterior 
fissure,  and  it  is  held  by  some  authorities  that  this  septum  is 


Hctodenn. 


Spinal  ganglion 


Outer  medullary  zone.        Central  canal.     Notochord.     Ventral  commissure. 
Fig.  125.— Transverse  section  of  developing  spinal  cord  of  a  twenty-two-day  sheep- 
embryo  (Bonnet). 


formed  by  the  growing  together  of  the  walls  of  the  dorsal 
part  of  the  central  canal. 

The  fiber-tracts  or  white  matter  of  the  spinal  cord  develop 
in  the  outer  or  neuroglia  layer,  each  fiber  being  the  elongated 
neurit  of  a  nerve-cell.  Some  of  the  fibers  originate  from  the 
nerve-cells  of  the  cord  while  others  grow  into  the  cord  from 
the  ganglia  of  the  posterior  roots  of  the  spinal  nerves.  The 
latter  method  of  origin  applies  especially  to  the  fibers  of  the 
tracts  of  Burdach  and  of  Goll. 

As  the  walls  of  the  neural  canal  thicken  through  the  mul- 
tiplication of  the  cells,  the  cavity  of  the  tube  is  gradually 


262  TEXT-BOOK  OF  EMBRYOLOGY. 

encroached  upon  almost  to  obliteration.  When  development 
is  complete,  all  that  remains  of  the  cavity  is  the  small  central 
canal  of  the  spinal  cord. 

The  length  of  the  spinal  cord  in  the  fourth  fetal  month 
corresponds  with  that  of  the  spinal  column,  its  lower  termi- 
nation being  opposite  the  last  coccygeal  vertebra.  From  this 
time  forward,  however,  the  cord  grows  less  rapidly  than  does 
the  spinal  column,  so  that  at  birth,  the  cord  terminates  at  the 
last  lumbar  vertebra,  and  in  adult  life  at  the  second  lumbar 
vertebra.  This  gradually  acquired  disproportion  in  the 
length  of  the  two  structures  explains  the  more  oblique 
direction  of  the  lower  spinal  nerves  as  compared  with  those 
higher  up.  In  the  early  condition  of  the  cord,  each  pair  of 
nerves  passes  almost  horizontally  outward  to  the  correspond- 
ing intervertebral  foramina,  but  as  the  spinal  column  gradu- 
ally outstrips  the  cord  in  growth,  the  lower  nerves  necessarily 
pursue  a  successively  more  oblique  course  to  reach  their 
foramina,  the  lower  nerves  being  almost  vertical  in  direction 
and  constituting,  collectively,  the  cauda  equina. 

THE  DEVELOPMENT  OF  THE  BRAIN. 

The  encephalic  portion  of  the  neural  tube — that  part 
devoted  to  the  production  of  the  brain — after  undergoing 
dilatation,  becomes  marked  off  into  the  three  primary  brain- 
vesicles,  the  fore-brain,  the  mid-brain  and  the  hind-brain,  by 
constrictions  in  the  lateral  walls  of  the  tube  (Fig.  126). 
This  division  occurs  at  an  early  stage,  before  the  closure  of 
the  tube  is  everywhere  complete.  The  vesicles  communicate 
with  each  other  by  rather  wide  openings.  As  in  the  spinal 
part  of  the  neural  canal,  the  walls  of  the  primary  brain-vesi- 
cles consist  of  epithelial  cells,  and  it  is  by  the  multiplication 
of  these  cells  in  unequal  degree  in  different  regions,  and  by  the 
formation  of  folds  in  certain  localities  that  the  various  parts 
of  the  adult  brain  are  developed  from  these  simple  epithelial 
sacs. 

The  stage  of  three  vesicles  is  soon  succeeded  by  the  five- 
vesicle  stage,  the  primary  fore-brain  vesicle  undergoing  divi- 


THE  DEVELOPMENl    OF  THE  BRAIN. 


263 


sion  into  two,  the  secondary  fore-brain  and  the  inter-brain, 
and  the  primary  hind-brain  vesicle  likewise  dividing,  a  little 
later,  into  the  secondary  bind- 
brain  and  the  after-brain. 

The  division  of  the  primary 
fore-brain  is  preceded  by  the 
appearance  upon  each  of  its 
lateral  walls  of  a  small  bulged- 
out  area  which  soon  assumes  the 
form  of  a  distinct  diverticulum. 
This  is  the  optic  vesicle,  the  ear- 
liest indication  of  the  develop- 
ment of  the  eye  (Fig.  126).  In 
the  further  process  of  growth 
the  base  of  attachment  of  the 
optic'  vesicle  becomes  lengthened 
out  into  a  relatively  slender  ped- 
icle, which  remains  in  connec- 
tion with  the  lower  part  of  the 
lateral  wall  of  the  brain-vesicle. 
Following  the  appearance  of  the 
optic  vesicle,  the  anterior  wall  of 
the  primary  fore-brain  vesicle 
projects  as  a  small  evagination,  which  latter  is  then  distinctly 
marked  oflP  from  the  parent  vesicle  by  a  groove  on  either  side. 
This  anterior  diverticulum  is  the  secondary  fore-brain  vesicle 
or  the  vesicle  of  the  cerebrum,  and  the  orignal  or  primary 
fore-brain  vesicle  is  now  the  vesicle  of  the  inter-brain. 

The  division  of  the  primary  hind-brain  is  effected  by  the 
development  of  a  constriction  of  its  lateral  wall,  this  resulting 
in  the  production  of  the  secondary  hind-brain  or  the  vesicle 
of  the  cerebellum,  and  the  after-brain  or  the  vesicle  of  the 
medulla  oblongata. 

While  the  three  primary  vesicles  at  first  lie  in  the  same 
straight  line,  they  begin  to  alter  their  relative  positions 
shortly  before  division.  The  change  of  position  is  coincident 
with  the  flexures  of  the  body  of  the  embryo  that  occur  at  this 
time.     Three  well-marked  flexures  appear,  the  result  being 


Anterior  brain-vesicle. 

Middle  brain-vesicle. 
Posterior  brain-vesicle. 

Fore-brain. 
Primary  optic  vesicle. 

Stalk  of  optic  vesicle. 
Inter-brain. 
Mid-brai?t. 
Hind-brain. 

After-brain. 
Fore-brain. 

Primary  optic  vesicle. 

Inter-brain. 

Mid-brain. 

Hind-brain. 

After-brain. 

Fig.  126. — Diagrams  illustrating 
the  primary  and  secondary  seg- 
mentation of  the  brain-tube  (Bon- 
net) . 


264 


TEXT-BOOK  OF  EMBRYOLOGY. 


that  the  fore-brain  is  bent  over  ventrad  to  a  marked  degree. 
The  most  anterior  of  these  flexures,  and  the  first  to  develop, 
is  the  so-called  cephalic  flexure  (Fig.  127),  the  primary  fore- 
brafn,  in  the  advanced  state  of  the  curvature,  being  bent 


Inter-brain. 


Fore-brain 


Cephalic  fexnre. 


Mid-brain. 


Hind-brain. 


After-brain. 


Cerebral  portion  of        Pontine 
pituitary  body.  flexure. 

Fig.  127.— Diagram  showing  relations  of  brain-vesicles  and  flexures  (Bonnet). 


around  the  termination  of  the  chorda  dorsalis  so  as  to  form  a 
right  angle,  and  later,  after  its  division,  an  acute  angle  with  the 
floor  of  the  mid-brain.  This  curvature  makes  the  mid-brain 
very  prominent  as  regards  the  surface  of  the  embryonic  body, 
producing  the  parietal  elevation  or  the  prominence  of  mid-brain. 

In  the  region  of  the  future  pons  Varolii,  on  the  floor  or 
ventral  wall  of  the  secondary  hind-brain,  is  a  second  well- 
marked  angularity.  This  is  the  pontal  flexure.  Its  con- 
vexity projects  forward. 

A  third  bend,  the  nuchal  flexure,  is  a  less  pronounced 
curvature  at  the  juncture  of  the  after-brain  with  the  spinal 
part  of  the  neural  tube. 

The  Metamorphosis  of  the  Fifth  Brain-vesicle.— 
The  fifth  brain-vesicle,  the  caudal  division  of  the  primary 
hind-brain,  differentiates  into  the  structures  which  surround 
the  lower  half  of  the  fourth  ventricle,  these  structures  con- 
stituting the  metencephalon  (Fig.  128).  The  histological 
changes  correspond  essentially  with  those  that  occur  in  the 
spinal  segment  of  the  neural  tube,  the  nerve-cells  and  fibers 


THE  DEVELOPMENT  OF  THE  BRAIN. 


265 


and  the  neuroglia  resulting  from  the  diiferentiation  of  the 
original  ectodermic  epithelium  of  which  the  wall  of  the  tube 
is  composed,  and  the  connective-tissue  stroma  growing  into 
these  from  the  surrounding  mesoderm. 

There  is  a  marked  disproportion  between  the  rate  of  growth 
of  the  tube  in  different  parts  of  its  circumference.  The 
great  thickening  of  the  ventral  and  lateral  walls  produces  the 
several  parts  of  the  medulla  oblongata.     In  the  dorsal  wall 


M  Ventricle  ^~/'\,^ 


Fig.  128.— Diagram  of  a  sagittal  section  of  the  brain  of  a  mammal,  showing 
the  type  of  structure  and  the  parts  that  develop  from  the  several  brain-vesicles 
(modified  from  Ediuger). 

growth  occurs  to  such  slight  extent  that  the  wall  in  this 
region  remains  a  thin  layer  of  epithelium.  As  a  consequence, 
the  cavity  of  the  neural  tube  is  not  encroached  upon  on  its 
dorsal  side  and  the  central  canal  of  the  spinal  cord  therefore 
expands  in  the  metencephalon  into  a  much  larger  space,  the 
lower  half  of  the  future  fourth  ventricle.  This  relative  ex- 
pansion of  the  central  canal  begins  to  be  apparent  in  the  third 
week  in  the  human  embryo,  from  which  period  it  continues 
to  increase.  A  cross-section  through  the  lower  part  of  the 
developing  medulla  shows  a  cavity  which  is  narrow  laterally 
but  w^hich  has  a  considerable  anteroposterior  extent.  A  sec- 
tion at  a  higher  level  discloses  a  triangular  space,  the  base  of 
the  triangle  being  the  dorsal  wall  of  the  cavity. 


266 


TEXT-BOOK  OF  EMBRYOLOGY. 


At  the  time  when  the  cavity  of  the  after-brain  acquires  a 
distinctly  triangular  shape — about  the  third  week — each  thick- 
ened lateral  half  of  the  tube  is  divisible  into  a  ventral  and  a 
dorsal  segment,  these  being  known  respectively  as  the  basal 
lamina  and  the  alar  lamina  (Fig.  129). 

The  first  indication  of  the  longitudinal  fiber-tracts  of  the 
medulla  is  presented  by  two  bands  of  fibers  which  appear  upon 
the  surface  of  the  alar  lamina  and  which 
constitute  the  ascending  root  of  the 
fifth  nerve  and  the  ascending  root  (funi- 
culus solitarius)  of  the  vagus  and  glosso- 
pharyngeal nerves.  These  are  later  cov- 
ered in  by  the  folding  over  of  the  dor- 
sal part  of  the  alar  lamina  (Fig.  130)  and 
thus  come  to  occupy  their  permanent 
position  in  the  interior  of  the  medulla. 
The  parts  of  the  alar  laminae  that  are 
folded  over  in  the  manner  referred 
to  differentiate  for  the  most  part 
into  the  restiform  bodies  or  inferior 
peduncles.  These  are  distinguishable  in  the  third  month. 
The  anterior  pyramidal  tracts  develop  from  the  ventral  parts 
of  the  basal  laminae  and  are  recognizable  in  the  fifth  month. 


Fig.  129.  —  Section 
through  upper  part  (cere- 
bellar region)  of  the 
fourth  ventricle  of  an 
embryo  (His) :  r,  roof  of 
neural  canal :  at,  alar 
lamina ;  bl,  basal  lamina ; 
V,  ventral  border. 


Fig.  130.— Section  across  the  lower  half  of  the  fourth  ventricle  of  an  embryo, 
showing  gradual  opening  out  of  the  neural  canal,  and  the  commencing  folding 
over  of  the  alar  lamina  at/  (His) :  v,  ventral  border;  t,  tenia;  ot,  otic  vesicle  ;  7'1, 
recessus  labyrinth i. 


Coincidentally  with   the   formation   of  the   fibers,   the  gray 
matter  of  the  medulla  assumes  its  permanent  form  and  arrange- 


THE  DEVELOPMENT  OF  THE  BRAIN.  267 

raent.  This  gray  matter,  although  in  part  peculiar  to  the 
medulla,  is  in  great  measure  but  the  continuation  of  the  gray 
matter  of  the  spinal  cord  rearranged  and  diflFerently  related 
because  of  the  motor  and  sensory  decussations  and  of  the  dor- 
sal expansion  of  the  central  canal.  A  notable  feature  of  this 
rearrangement  is  the  presence  of  masses  of  gray  matter  im- 
mediately beneath  the  floor  or  ventral  wall  of  the  now  ex- 
panded cavity  or  fourth  ventricle. 

As  stated  above,  the  dorsal  wall  of  the  after-brain  vesicle 
remains  an  extremely  thin  epithelial  lamina,  and  the  cavity 
in  consequence  expands  toward  the  dorsal  surface.  Owing 
to  the  excessive  delicacy  of  this  dorsal  wall  of  the  cavity,  it 
is  easily  destroyed  in  dissection,  with  the  effect  of  disclosing 
a  triangular  fossa  (Fig.  135)  on  the  dorsal  surface  of  the 
medulla,  which  in  connection  with  a  similar  depression  on 
the  dorsal  surface  of  the  pons,  constitutes  the  rhomboidal  fossa, 
or  the  fourth  ventricle  of  the  brain. 

It  is  often  stated  in  descriptions  of  the  medulla  and  fourth 
ventricle  that  the  latter  is  produced  by  the  opening  out  of 
the  central  canal  of  the  cord  to  the  dorsal  surface.  It  should 
be  borne  in  mind,  however,  that  the  central  canal  does  not, 
in  reality,  open  out  to  the  surface,  although  it  may  appear  to 
do  so  because  of  the  attenuated  condition  of  its  dorsal  bound- 
ary. The  thin  epithelial  roof  or  dorsal  wall  of  the  after- 
brain  becomes  adherent  to  the  investing  layer  of  pia  mater, 
thus  forming  the  tela  choroidea  inferior,  which  roofs  over 
the  lower  half  of  the  fourth  ventricle  (Fig.  128).  The  pia 
mater  invaginates  the  epithelial  layer  to  form  the  choroid 
plexuses  of  the  fourth  ventricle.  Although  apparently 
within  the  cavity  of  the  ventricle,  the  choroid  plexuses 
are  excluded  from  it  by  the  layer  of  epithelium,  the  mor- 
phological roof  of  the  after-brain,  which  they  have  pushed 
before  them. 

While,  for  the  most  part,  the  roof  of  the  after-brain  con- 
sists of  the  thin  epithelial  layer  referred  to  above,  there  are 
slight  linear  thickenings,  the  ligulse,  along  its  lateral  margins, 
and  at  its  lower  angle,  the  obex.  At  the  upper  margin  of  the 
roof,  at  the  place  of  junction  with  the  hind-brain,  there  is 


268  TEXT-BOOK  OF  EMBRYOLOGY. 

also  a  thickened  area,  the  inferior  medullary  velum.  These 
regions  of  thicker  tissue  serve  to  eifect  the  transition  from 
the  thin  epithelial  layer  that  helps  to  form  the  inferior 
choroidal  tela  to  the  more  massive  boundaries  of  the  rhom- 
boidal  fossa. 

The  Hind-brain  Vesicle  or  Kpencephalon. — The 
epencephalon  consists  of  the  pons,  the  cerebellum  with  its 
superior  and  middle  peduncles,  and  the  valve  (valve  of 
Vieussens).  These  structures  are  produced  by  the  thicken- 
ing of  the  walls  of  the  fourth  or  hind-brain  vesicle. 

The  pons  is  formed  by  the  thickening  of  the  ventral  wall 
of  the  vesicle.  Its  transverse  fibers  become  recognizable 
during  the  fourth  month. 

The  cerebellum  grows  from  the  posterior  part  of  the  roof 
or  dorsal  wall  of  the  vesicle  (Fig.  128).  The  first  indication 
of  its  development  is  seen  as  a  thick  transverse  ridge  or 
fold  on  the  posterior  extremity  of  the  dorsal  wall  (Figs. 
131,  132).  In  the  third  month  the  central  part  of  this 
ridge,  now  grown  larger,  presents  four  deep  transverse 
grooves  with  the  result  of  dividing  the  original  eminence 
into  five  transverse  ridges.  The  grooved  part  of  the  ridge 
is  the  portion  that  subsequently  becomes  the  vermiform 
process  or  median  lobe  of  the  cerebellum,  while  the  smooth 
-lateral  portions  become  the  lateral  hemispheres.  As  the 
vermiform  process  increases  in  bulk,  two  of  the  ridges  come 
to  lie  upon  its  upper  surface  and  three  on  the  inferior  aspect. 
These  ridges  and  furrows  persist  throughout  life  as  the 
principal  convolutions  and  fissures  of  the  vermiform  proc- 
ess (Figs.  133,  134). 

The  lateral  parts  of  the  primary  ridge  increase  in  size  and 
eventually,  in  the  human  brain,  outstrip  the  median  lobe  in 
growth.  They  acquire  their  chief  transverse  fissures  in  the 
fourth  or  fifth  month,  and  the  smaller  sulci  later. 

The  thickened  cerebellar  ridge  on  the  roof  of  the  hind- 
brain  vesicle  being  continuous  with  the  lateral  walls,  the 
continuity  of  the  cerebellar  hemispheres  with  the  pons 
through  the  middle  and  superior  cerebellar  j)eduncles  and 
with  the  medulla  by  means  of  the  inferior  peduncles,  is  easily 


THE  HIND-BRAIN  VESICLE  OR  EPENCEPHALON.    269 

understood.  These  bands  of  fibers  become  evident,  the  in- 
ferior in  the  third  month,  the  middle  in  the  fourth  month, 
and  the  superior  in  the  fifth  month. 

While  the  posterior  part  of  the  roof  of  the  hind-brain 
thickens  and  develops  into  the  cerebellum,  all  the  remaining 
part  of  this  roof  remains  relatively  thin  and  becomes  the 


ib  mb      is  lib  IVv 


in  m    pf 


ol  o  pf    0 


mb 


hb 


p      pf  m 

Fig.  131. — Brains  of  human  embryos,  from  reconstructions  by  His:  A,  brain 
from  fifteen-day  embryo ;  B,  from  three-and-a-half-week  embryo  ;  C,  from  seven- 
and-a-half-week  fetus:  /6,  ib,  mb,  hb,  ab,  fore-,  inter-,  mid-,  hind-,  and  after-brain 
vesicles  ;  o,  optic  vesicle ;  ov,  otic  vesicle ;  in,  infundibulum  ;  m,  mammillary  proc- 
ess ;  pf,  pontine  flexure ;  IVv,  fourth  ventricle ;  nk,  cervical  flexure ;  ol,  olfactory 
lobe ;  h,  basilar  artery  ;  p,  pituitary  recess. 

anterior  medullary  velum  or  the  valve  of  Vieussens  (Fig.  1 28). 
The  relations  of  this  structure  in  the  mature  brain,  stretching 
across,  as  it  does,  from  one  superior  cerebellar  peduncle  to 
the  other  and  being  continuous  posteriorly  with  the  white 
matter  of  the  cerebellum,  are  easily  explained  in  the  light  of 
the  fact  that  all  these  parts  are  but  the  specialized  dorsal  and 


270  TEXT-BOOK   OF  EMBRYOLOGY. 

lateral  walls  of  ihe  hind-brain  vesicle.  Since  the  roof  of  the 
hind-brain  vesicle  is  continuous  with  that  of  the  after-brain 
or  fifth  vesicle,  it  will  be  seen  that  the  cerebellum  must  be  in 
continuity  with  the  roof  of  the  medullary  part  of  the  fourth 
ventricle.  The  transition  from  the  cerebellum  to  the  epi- 
thelium of  the  tela  choroidea  inferior  is  effected  by  a  pair  of 
thin  crescent-shaped  bands  of  white  nerve-matter  which  pass 
downward  from  the  central  white-matter  of  the  cerebellum, 
and  which  are  collectively  known  as  the  inferior  or  posterior 
medullary  velum.  Thus,  as  the  result  of  unequal  growth, 
there  are  produced  from  the  continuous  dorsal  walls  of  the 
fourth  and  fifth  vesicles  the  thin  laminar  medullary  velum 
or  valve,  the  massive  cerebellar  lobes,  the  thin  bands  known 
as  the  inferior  medullary  velum,  and  the  single  layer  of  epi- 
thelium which,  with  a  layer  of  pia  mater,  constitutes  the 
inferior  choroidal  tela. 

Although  the  fourth  and  fifth  brain-vesicles  are  at  first 
delimited  from  each  other  by  a  constriction,  this  constriction, 
as  development  goes  on,  disappears,  the  cavity  of  the  fourth 
vesicle  and  that  of  the  fifth  together  constituting  the  fourth 
ventricle  of  the  brain. 

The  walls  of  the  fourth  or  hind-brain  vesicle  then  give 
rise  ventrally  to  the  pons,  laterally  to  the  superior  and  mid- 
dle cerebellar  peduncles,  and  dorsally  to  the  valve  and  the 
cerebellum,  while  its  cavity  becomes  the  anterior  half  of  the 
fourth  ventricle. 

The  Mid-brain  Vesicle. — The  third  brain-vesicle  or 
the  vesicle  of  the  mid-brain  or  mesencephalon  gives  rise  to 
the  structures  surrounding  the  aqueduct  of  Sylvius,  the  per- 
sistent part  of  the  cavity  constituting  the  aqueduct  itself. 

The  thickening  of  the  ventral  wall  of  the  vesicle  results  in 
the  formation  of  the  crura  cerebri  and  the  posterior  perforated 
lamina  or  space  included  between  them.  The  crura  first 
become  apparent  in  the  third  month  as  a  pair  of  rounded 
longitudinal  ridges  on  the  ventral  surface  of  the  vesicle. 
These  remain  relatively  small  until  the  fifth  month,  when 
the  longitudinal  fibers  of  the  pons  begin  to  grow  into  them. 
After  this  occurrence  their  increase  in  size  is  comparatively 


THE  MID-BRAIN   VESICLE.  'Z7i 

rapid,  their  ventral  parts  or  crustae  becoming  separated  from 
each  other  and  including  between  them  the  posterior  per- 
forated lamina. 

The  roof  or  dorsal  wall  of  the  mid-brain  vesicle  under- 
goes considerable  thickening  (Fig.  131),  especially  in  the 
Sauropsida  (birds,  reptiles,  fishes).  In  the  fifth  week  a  longi- 
tudinal ridge  appears  upon  the  dorsal  wall,  which  in  the  third 
month  is  replaced  by  a  furrow.  The  expansion  of  the  wall 
on  each  side  of  the  furrow  produces  a  pair  of  rounded  emi- 
nences (Figs.  132-135),  which,  in  birds,  attain  to  a  much 


B 
Fig.  132.— Brain  of  human  fetus  of  about  eight  weeks :  A,  enlarged  ;  B,  actual 
size;  i^'ft,  fore-brain  ;   7b,  inter-brain  ;  J/6,  mid-brain  ;  Jifft,  hind-brain ;  ^6,  after- 
brain  ;  P,  folds  of  pia  mater. 

greater  development  than  in  mammals  and  constitute  the 
corpora  bigemina  or  optic  lobes.  In  the  human  embryo,  each 
of  these  elevations  is  divided  into  two  by  an  oblique  groove, 
and  thus  are  formed  the  corpora  quadrigemina,  Avhich  are 
peculiar  to  man  and  other  mammals. 

~  The  part  of  the  dorsal  wall  of  the  vesicle  that  underlies 
the  corpora  quadrigemina  is  the  lamina  quadrigemina. 

The  thickening  which  the  walls  of  the  vesicle  undergo  to 
produce  the  several  parts  of  the  mid-brain  encroaches  so 
much  upon  its  cavity  that  an  exceedingly  small  canal,  the 
aqueduct  of  Sylvius,  remains.  It  is  scarcely  necessary  to 
point  out  that  this  canal  is  a  part  of  the  ventricular  system 
of  the  brain,  establishing  a  communication  between  the  fourth 
ventricle  and  the  third  ventricle  or  cavity  of  the  inter-brain. 


272 


TEXT-BOOK  OF  EMBRYOLOGY. 


The  Metamorphosis  of  the  Inter-brain  Vesicle. — 

The  inter-brain  vesicle  results  from  the  division  of  the  pri- 
mary fore-brain  vesicle,  comprising  what  is  left  of  the  latter 
after  the  outgrowth  from  it  of  the  diverticulum  that  becomes 
the  secondary  fore-brain.  The  thickening  of  the  walls  of 
the  inter-brain  vesicle  produces  the  structures  which  surround 
the  third  ventricle  in  the  mature  condition,  and  which  consti- 
tute collectively  the  thalamenceplialon  or  inter-brain,  the  cavity 
of  the  vesicle  persisting  as  the  adult  third  ventricle.  These 
structures  are  the  optic  thalami,  which  are  formed  from  the 
lateral  walls;  the  velum  interpositum  and  the  pineal  body, 
which  develop  from  the  roof;  and  the  lamina  cinerea,  the 


mo    p  olf  est  olf 

Fig.  133.— A,  mesial  section  through  brain  of  a  human  fetus  of  two-and-a-half 
months  (His) :  ch,  cerebral  hemisphere  ;  o,  optic  thalamus ;  /m,  foramen  of  Monro ; 
olf,  olfactory  lobe ;  p,  pituitary  body ;  mo,  medulla  oblongata ;  cq.  corpora  quadri- 
gemlna;  c6,  cerebellum.  B,  brain  of  human  fetus  of  three  months  (His):  olf, 
olfactory  lobe;  est,  corpus  striatum;  cq,  corpora  quadrigemina ;  c6,  cerebellum; 
mo,  medulla  oblongata. 

tuber  cinereum,  the  infundibulum,  the  posterior  lobe  of  the 
pituitary  body  and  the  corpora  albicantia,  which  are  differ- 
entiated from  the  floor  of  the  vesicle. 

The  lateral  walls  of  the  vesicle  undergo  the  most  marked 
thickening.  The  cell-multiplication  here  is  so  rapid  that 
each  lateral  wall  is  converted  into  a  large  ovoid  mass  of  cells 
with  intermingled  bands  of  fibers,  the  optic  thalamus. 

The  roof  of  the  inter-brain  vesicle,  in  notable  contrast 
with  the  lateral  walls,  remains  extremely  thin   throughout 


METAMORPHOSIS  OF  THE  INTER-BRAIN   VESICLE.   273 

the  greater  part  of  its  extent  (Fig.  128).  From  the  back 
part  of  the  roof,  at  a  point  immediately  in  front  of  the 
lamina  quadrigemina  of  the  mid-brain,  a  diverticulum  grows 
out  and  becomes  metamorphosed  into  the  pineal  body.  With 
this  exception,  the  roof  of  the  vesicle  remains  a  single  layer 
of  epithelium,  just  as  in  the  case  of  the  roof  of  the  after- 
brain.  This  epithelial  layer  adheres  closely  to  the  pia  mater, 
which  covers  it  in  common  with  the  other  parts  of  the  brain. 
As  the  fore-brain  expands,  it  covers  the  inter-brain,  the 
under  surface  of  the  cerebral  hemispheres  of  the  former 
being  closely  applied  to  the  roof  of  the  latter.  As  a  con- 
sequence, the  pia  mater  on  the  under  surface  of  the  fore- 
brain  is  brought  into  contact  with  and  adheres  to  the  pia 
covering  the  roof  of  the  inter-brain.  Thus  the  thin  epithelial 
roof  of  the  inter-brain  becomes  closely  united  with  the  two 
layers  of  the  pia  mater  to  form  the  velum  interpositum  or 
tela  choroidea  anterior  or  superior  of  adult  anatomy.  Ob- 
viously, the  edges  of  the  velum  interpositum  rest  upon  the 
optic  thalami,  and  its  piamatral  layers  are  continued  into  the 
cavities  of  the  lateral  ventricles  (Fig.  134).    The  space  occu- 


PiG.  134— Brain  of  fetus  of  three  months,  enlarged.  The  outer  wall  of  the  right 
hemisphere  has  been  removed;  LH,  left  hemisphere;  CS,  part  of  corpus  striatum; 
FS,  site  of  fossa  of  Sylvius ;  V,  vascular  fold  of  pia  mater  which  has  been  invag- 
Inated  through  the  mesial  wall  of  the  hemisphere;  Mb,  mid-brain;  C,  cerebellum; 
M,  medulla  oblongata. 


pied  by  the  velum  is  designated  the  transverse  fissure  of  the 
brain,  and  it  is  often  stated  that  the  pia  mater  is  pushed  in 
from  behind,  between  the  optic  thalami  and  the  cerebral  hemi- 


274  TEXT-BOOK  OF  EMBRYOLOGY. 

spheres.  As  will  be  seen  from  the  above  description,  how- 
ever, its  development  really  begins  in  front. 

The  pineal  gland  or  conarium  develops  from  the  back  part 
of  the  roof  of  the  inter-brain  at  its  point  of  junction  with 
the  mid-brain  (Fig.  128).  This  body  is  found  in  all  ver- 
tebrate animals  except  the  amphioxus,  but  its  form  varies 
greatly  in  different  groups.  In  all  cases  it  begins  as  a  small 
pouch-like  evagination  from  the  roof  of  the  inter-brain,  the 
diverticulum  being  directed  forward.  In  the  human  brain 
alone  the  structure  is  subsequently  directed  backward,  so 
that  it  comes  to  occupy  a  position  just  over  the  corpora 
quadrigemina.  This  peculiarity  of  location  is  due  probably 
to  the  greater  development  of  the  human  corpus  callosum, 
by  which  the  conarium  is  crowded  backward. 

In  selachians  (sharks  and  dog-fish),  the  enlarged  vesicular 
end  of  the  diverticulum,  which  is  lined  with  ciliated  columnar 
cells,  lies  outside  the  cranial  capsule  and  is  connected  with 
the  inter-brain  by  the  long  hollow  stalk  which  perforates 
the  roof  of  the  cranium.  In  many  reptiles,  the  conarium  is 
more  highly  specialized.  In  the  chameleon,  for  example,  the 
peripheral  extremity  has  the  form  of  a  small  closed  vesicle 
which  lies  outside  the  roof  of  the  cranium  and  which  is 
covered  by  a  transparent  patch  of  skin.  The  stalk  in  this 
case  is  partly  a  solid  cord  and  partly  a  hollow  canal,  which 
latter  opens  into  the  cavity  of  the  inter-brain.  The  solid 
portion  lies  within  a  foramen  in  the  parietal  bone,  the  parietal 
foramen.  A  further  modification  of  the  conarium  is  presented 
in  lizards,  blind-worms,  and  some  other  reptiles.  In  these 
the  vesicle  undergoes  a  marked  specialization,  its  peripheral 
wall  being  so  modified  as  to  become  transparent  and  to  re- 
semble the  crystalline  lens  of  the  eye,  while  the  opposite 
deeper  wall  comes  to  consist  of  several  layers  of  cells — some 
of  which  become  pigmented — and  acquires  a  striking  resem- 
blance to  the  retina.  The  stalk  of  the  body,  Avhich  perforates 
the  roof  of  the  skull  and  is  attached  to  the  roof  of  the  inter- 
brain,  bears  a  certain  likeness  to  the  optic  nerve,  being  solid 
and  composed  of  fibers  and  elongated  cells.  The  presence 
of  the  transparent  epidermal  plate  which  covers  the  vesicle 


METAMORPHOSIS  OF  THE  INTER-BRAIN   VESICLE.   275 

serves  to  complete  the  similarity  of  this  particular  type  of 
pineal  body  to  the  eye  of  vertebrate  animals.  It  is  for  this 
reason  that  it  is  often  designated  the  pineal  or  parietal  eye 
and  that  it  has  been  looked  upon  as  a  third  or  unpaired 
organ  of  vision. 

In  man  and  other  mammals  and  in  birds  the  pineal  diver- 
ticulum does  not  reach  the  degree  of  development  that  is 
attained  in  certain  of  the  Reptilia.  The  evagination  from 
the  roof  of  the  inter-brain  begins  in  the  sixth  week  in  the 
human  embryo.  The  peripheral  end  of  the  process  enlarges 
somewhat  and  small  masses  of  cells  project  from  it  into  the 
surrounding  mesodermic  tissue.  These  cellular  outgrowths, 
giving  oflF  secondary  projections,  become  converted  into  small 
closed  follicles  lined  with  columnar  ciliated  cells.  The  folli- 
cles in  the  case  of  mammals  very  soon  become  solid  or  nearly 
so  by  the  accumulation  of  cells  in  their  interior.  Solid  con- 
cretions of  calcareous  matter,  the  so-called  brain-sand  (acer- 
vulus  cerebri)  are  found  in  the  follicles  in  the  adult.  By 
these  alterations  the  pineal  body  of  birds  and  mammals 
acquires  a  structure  resembling  that  of  a  glandular  organ. 
Since  it  is  only  the  end  of  the  diverticulum  that  becomes 
thus  altered,  the  remaining  part  constitutes  the  relatively 
slender  stalk  of  the  pineal  body,  the  stalk  being  solid  at 
maturity  except  at  its  point  of  attachment  to  the  inter-brain, 
where  a  portion  of  the  cavity  persists  as  the  pineal  recess  of 
the  third  ventricle. 

The  pineal  body  of  man  and  the  higher  vertebrates  is  there- 
fore a  rudimentary  structure  and  is  the  representative  of  an 
organ  that  is  much  more  highly  developed  in  some  of  the 
lower  members  of  the  same  series.  Its  true  significance  is 
still  a  matter  of  conjecture.  Although  resembling  the  eye  in 
its  structure,  and  although  regarded  by  some  on  that  account 
as  primitively  an  organ  of  vision,  it  is  considered  probable  by 
others  that  in  its  most  highly  developed  condition  it  is  an 
organ  of  heat  perception. 

The  floor  of  the  inter-brain  vesicle  presents  several  interesting 
metamorphoses.  The  anterior  part  of  the  floor  remains  quite 
thin  and  becomes  the  lamina  cinerea  of  the  mature  brain  (Fig. 


276  TEXT-BOOK  OF  EMBRYOLOGY. 

128).  Immediately  posterior  to  this  region,  the  floor  of  the 
vesicle  pouches  out,  this  evagination  developing  into  a  slender 
tube,  the  infundibulum.  Behind  the  point  of  origin  of  the 
infundibulum  a  second  protuberance  indicates  the  beginning 
of  the  tuber  cinereum.  By  subsequent  alterations,  the  tuber 
cinereum  enlarging  in  circumference  so  as  to  include  the 
point  of  origin  of  the  infundibulum,  the  base  of  attachment 
of  the  infundibulum  comes  to  be  the  center  of  the  tuber 
cinereum,  so  that  the  cavity  of  the  former  is  a  continuation  of 
the  cavity  of  the  latter.  The  end  of  the  infundibulum 
becomes  the  posterior  lobe  of  the  pituitary  body  or  hypo- 
physis (Figs.  128  and  133).  Posterior  to  the  tuber  cine- 
reum a  small  evagination  of  the  floor  of  the  vesicle 
appears  and  becomes  divided  in  the  early  part  of  the  fourth 
month  into  two  lateral  halves  by  a  median  furrow.  The 
two  little  bodies  thus  formed  become,  after  further  develop- 
ment, the  corpora  albicantia. 

The  hypophysis  or  pituitary  body  briefly  referred  to  above 
requires  more  extended  consideration  because  of  its  mor- 
phological importance.  The  posterior  lobe  of  this  body  is  the 
enlarged  end  of  the  infundibulum,  which  is  an  evagination  of 
the  floor  of  the  inter-brain.  The  cells  in  the  lower  end  of 
the  infundibulum  specialize  into  nerve-cells,  and  nerve- 
fibers  also  develop.  In  some  lower  vertebrates  these  ele- 
ments are  retained  throughout  life,  but  in  man  and  the 
higher-type  animals  the  distinctively  nervous  character  of 
the  tissues  is  soon  lost,  and  the  cavity  of  this  part  of  the 
infundibulum  suffers  obliteration.  The  branched  pigment- 
cells  sometimes  recognizable  in  the  posterior  lobe  of  the 
human  pituitary  body  are  the  only  remnant  of  the  early 
nerve-cells. 

The  anterior  lobe  of  the  hypophysis  is  essentially  different 
in  origin  as  well  as  in  structure  from  the  posterior  lobe.  It 
is  produced  by  an  evagination  from  the  posterior  wall  of  the 
primitive  pharynx,  but  from  that  region  of  the  pharynx  which 
is  anterior  to  the  pharyngeal  membrane  and  which  therefore 
belongs  to  the  primitive  mouth-cavity  (Fig.  55,  p.  119).  The 
out-pocketing  of  the   pharyngeal  wall  begins  in  the  fourth 


METAMORPHOSIS   OF  THE  INTER-BRAIN   VESICLE.    277 

week,  shortly  after  the  rupture  of  the  pharyngeal  membrane. 
The  little  pouch  is  the  pocket  of  Rathke.  The  pouch  grows 
upward  and  backward  toward  the  floor  of  the  inter-brain  and 
meets  the  end  of  the  infundibulum.  As  the  pharyngeal 
diverticulum  lengthens,  its  stalk  becomes  a  slender  duct, 
which  for  some  time  retains  its  connection  with  the  pharynx. 
As  the  membranous  base  of  the  skull  becomes  cartilaginous, 
the  duct  begins  to  atropliy,  and  finally  entirely  disappears. 
In  selachians,  however,  it  is  retained  permanently,  establish- 
ing thus  a  connection  between  the  hypophysis  and  the  pharyn- 
geal cavity.  With  the  disappearance  of  the  duct  the  enlarged 
extremity  of  the  diverticulum  becomes  a  closed  vesicle  lying 
now  within  the  cavity  of  the  brain-case,  in  contact  with  the 
end  of  the  infundibulum.  From  the  wall  of  the  vesicle  nu- 
merous little  tubular  projections  grow  out  into  the  enveloping 
mesodermic  tissue,  and  these,  by  detachment  from  the  parent 
vesicle,  become  closed  tubes  or  follicles.  The  entire  structure 
becomes  converted  in  this  manner  into  a  mass  of  closed  fol- 
licles held  together  by  connective  tissue,  after  which  event 
this  mass  acquires  intimate  union  with  the  infundibular  lobe. 

Thus  the  pituitary  body  consists  of  two  genetically  distinct 
parts,  the  anterior  lobe  being  derived  from  the  ectoderm  of 
the  primitive  pharyngeal  or  buccal  cavity,  and  the  posterior 
lobe  from  the  ectoderm  of  the  central  nervous  system.  The 
posterior  lobe,  developing  as  it  does  as  an  evagination  from 
the  floor  of  the  inter-brain,  is  to  be  regarded  as  a  small  out- 
lying lobe  of  the  brain. 

What  remains  of  the  cavity  of  the  inter-brain,  after  its 
walls  have  thus  developed  into  the  several  structures  de- 
scribed, is  the  third  ventricle  of  the  adult  brain,  and  the 
aperture  of  communication  with  the  secondary  fore-brain 
vesicles  becomes  the  foramen  of  Monro.  Since  the  lateral 
walls  become  the  massive  optic  thalami,  while  the  dorsal  and 
ventral  walls  give  rise  to  much  thinner  structures,  the  cavity 
of  the  vesicle  is  encroached  upon  to  a  greater  extent  on  the 
sides  than  from  above  and  below,  and  hence  the  form  of  the 
third  ventricle  in  the  mature  condition  is  that  of  a  narrow 
vertical  fissure  between  the  thalami. 


278  TEXT-BOOK  OF  EMBRYOLOGY. 

The  Metamorphosis  of  the  Fore-brain  Vesicle. — 

The  secondary  fore-brain  vesicle  gives  rise  to  the  prosen- 
cephalon, which  includes  the  cerebral  hemispheres  and  the 
structures  belonging  directly  to  them.  As  above  indicated, 
this  vesicle  grows  from  the  anterior  wall  of  the  primary  fore- 
brain  vesicle  as  a  diverticulum  which  is  at  first  single,  but 
which  soon  becomes  divided  into  two  lateral  halves  by  the 
formation  of  a  cleft  in  the  median  plane  (Fig.  131, /6).  This 
cleft,  or  interpallial  fissure  is  the  early  representative  of  the 
longitudinal  fissure  of  the  adult  cerebrum.  The  two  vesicles 
remain  attached  at  their  bases  or  stalks  with  the  parent  vesicle 
and  communicate  by  a  common  orifice  with  its  cavity.  The 
vesicles  of  the  secondary  fore-brain  grow  in  an  upward  and 
backward  direction  as  well  as  laterally,  and  their  develop- 
ment is  so  much  more  rapid  than  that  of  the  other  vesicles 
that  they  soon  spread  over  them  and  partially  hide  them 
from  view.  It  is  for  this  reason  that  the  mass  resulting 
from  the  fore-brain  vesicles,  except  their  basal  ganglia,  is 
known  in  comparative  anatomy  as  the  pallium  or  mantle 
(Fig.  128).  _ 

The  relative  rate  of  growth  of  the  cerebral  hemispheres  is 
such  that  in  the  third  month  they  completely  overlie  the 
inter-brain  and  by  the  sixth  month  they  have  extended  so 
far  back  as  to  hide  the  corpora  quadrigemina. 

The  mesodermic  tissue  surrounding  the  developing  brain 
becomes  differentiated  into  the  three  brain-memhranes,  which 
penetrate  into  the  fissure  and  therefore  invest  the  vesicles 
on  their  mesial  surfaces  as  well  as  elsewhere.  The  invag- 
inating  layers  of  the  dura  mater  constitute  the  primitive 
falx  cerebri. 

The  metamorphosis  of  this  pair  of  sacs  into  the  cerebral 
hemispheres  is  brought  about  by  three  important  processes  : 
first,  the  multiplication  of  the  cells  which  compose  its  walls 
to  form  the  masses  of  nerve-cells  and  fibers  of  the  hemi- 
spheres ;  second,  the  formation  of  folds  in  the  wall  whereby 
are  produced  the  fissures  which  divide  the  hemispheres  into 
lobes  and  convolutions ;  and  third,  the  development  of  adhe- 
sions within  certain  areas  between  the  mesial  walls  of  the 


METAMORPHOSIS   OF  THE  FORE-BRAIN    VESICLE.     279 

two  vesicles,  by  which  the  system  of  commissures  of  the 
hemispheres  is  produced. 

The  walls  of  the  cerebral  vesicles  are  at  first  very  thin, 
consisting  merely  of  several  layers  of  spindle-shaped  cells. 
By  the  rapid  multiplication  of  these  cells,  the  walls  are  thick- 
ened and  the  cavity  of  the  vesicle  is  gradually  encroached 
upon  until  the  mature  condition  of  the  brain  is  attained, 
when  the  cavity  is  relatively  very  much  smaller  than  in  the 
fetus  and  constitutes  the  ventricle  of  the  hemisphere  or  the 
lateral  ventricle.  The  nerve-cells  develop  processes  or  polar 
prolongations,  of  which  the  most  conspicuous,  the  axis-cylin- 
der processes,  lengthen  out  to  form  the  axis  cylinders  of 
nerve-fibers.  The  fibers  thus  formed  are  directed  away  from 
the  surface  and  make  up  the  white  medullary  matter  of  the 
hemispheres,  while  the  more  superficially  placed  layers  of 
cells  constitute  the  gray  matter  of  the  cortex  of  the  brain. 

In  addition  to  the  cortical  or  superficial  gray  matter  there 
are  masses  of  gray  matter  within  the  hemisphere,  the  basal 
ganglia,  which  are  likewise  collections  of  nerve-cells.  Within 
a  limited  area  on  the  lateral  wall  of  each  cerebral  vesicle, 
near  the  lower  margin,  the  cells  undergo  excessive  prolifera- 
tion resulting  in  the  production  of  a  large  ganglionic  mass, 
the  corpus  striatum,  and  of  two  smaller  aggregations  of  cells, 
the  claustrum  and  the  nucleus  amygdalae.  These  basal  ganglia 
are  in  reality  an  infolded  part  of  the  cortex. 

Inasmuch  as  the  cortical  matter  develops  more  rapidly,  as 
regards  superficial  extent,  than  does  the  medullary  substance, 
the  cortex  becomes  thrown  into  folds,  forming  thus  the  con- 
volutions and  fissures  of  the  hemispheres. 

Some  of  the  fissures  of  the  brain  are  produced  by  an  in- 
folding of  the  entire  thickness  of  the  vesicle-wall  so  that 
their  presence  is  indicated  by  corresponding  projections  in 
the  walls  of  the  ventricles.  Such  fissures  are  distinguished 
as  total  fissures.  Included  in  this  category  are  the  fissure 
of  Sylvius,  which  is  represented  in  the  wall  of  the  lateral 
ventricle  by  the  corpus  striatum  ;  the  calcarine  fissure,  the 
dentate  fissure,  and  the  collateral  fissure,  which  are  responsible 
respectively  for  the  calcar  avis,  the  hippocampus  major,  and 


280 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  collateral  eminence  of  the  lateral  ventricle  ;  and  the  great 
transverse  fissure  of  the  brain,  the  infolded  wall  in  this  case 
being  very  thin  and  consisting  merely  of  the  layer  of  epi- 
thelium which  covers  the  choroid  plexus. 

The  fissure  of  Sylvius  is  the  earliest  fissure  formed  and  one 
of  the  most  important.  At  an  early  period  in  the  history 
of  the  secondary  fore-brain,  there  is  a  region  in  the  lower 
part  of  the  lateral  wall  of  the  vesicle  where  expansion  is 
less  rapid  than  elsewhere,  this  area,  as  it  were,  remaining 
fixed.      As   the  vesicle-wall    immediately  surrounding  this 


Fig.  135.— Posterior  view  of  brain  sliown  in  Fig.  136 :  A,  actual  size ;  B,  en- 
larged ;  Mb,  mid-brain  ;  C,  cerebellum ;  RF,  rhomboidal  fossa  (its  dorsal  wall  having 
been  removed) ;  M,  medulla  oblongata. 

spot  continues  to  expand,  a  dimpling  of  the  wall  is  produced, 
the  depression  being  designated  the  fossa  of  Sylvius  (Fig.  136, 
S).  The  part  of  the  vesicle-wall  behind  the  fossa  advances 
forward  and  downward  to  form  the  future  temporal  lobe,  and 
thus  the  fossa  comes  to  be  surrounded  by  a  convolution 
having  the  form  of  an  incomplete  ring,  open  in  front — the 
ring  lobe.  The  floor  of  the  fossa  undergoes  very  consider- 
able thickening  to  form  the  basal  ganglia — that  is,  the  corpus 
striatum,  the  amygdaloid  nucleus,  and  the  claustrum.  These 
structures,  most  conspicuously  the  corpus  striatum,  encroach 
upon  the  cavity  of  the  vesicle,  the  nucleus  caudatus  of  the 


METAMORPHOSIS  OF  THE  FORE-BRAIN  VESICLE.     281 

corpus  striatum  bulging  into  the  floor  and  outer  wall  of  the 
adult  lateral  ventricle. 

H 


Fig.  136.— Brain  of  human  fetus  of  approximately  three  months :  A,  enlarged ; 
B,  actual  size;  H,  hemisphere;  Mh,  mid-brain;  C,  cerebellum;  M,  medulla  ob- 
longata ;  S,  fossa  of  Sylvius. 

The  cortical  matter  of  the  floor  of  the  fossa  of  Sylvius, 
being  circumscribed  by  a  groove  or  sulcus,  constitutes  the 


Fig.  137.— Brain  of  human  fetus  of  three  months,  with  right  half  of  fore-brain, 
inter-brain,  and  mid-brain  removed :  1  b,  cavity  of  inter-brain ;  Hy,  site  of  hyp- 
ophysis ;  Mhr,  mid-brain  roof;  Mbc,  mid-brain  cavity ;  C,  cerebellum  ;  M,  medulla 
oblongata. 

central  lobe  or  island  of  Reil,  which  is  subsequently  broken 
up,  by  secondary  fissures,  into  from  five  to  seven  small  con- 
volutions. 


282  TEXT-BOOK  OF  EMBRYOLOGY. 

By  the  extension  of  the  fossa  of  Sylvius  backward,  and  by 
the  increased  growth  of  the  vesicle-wall  above  and  below  it, 
the  fossa  is  converted  into  the  fissure  of  Sylvius  (Fig.  140,  B), 
and  the  island  of  Reil  is  hidden  from  view.  Subsequently 
the  ascending  and  anterior  limbs  are  added  to  the  chief  or 
horizontal  part  of  the  fissure. 

The  anterior  part  of  the  ring  lobe  corresponds  with  the 
future  frontal  lobe,  the  posterior  part  represents  the  parietal 
lobe  while  the  lower  part  of  the  ring  becomes  the  temporal 
lobe.  A  backward  extension  of  the  ring  lobe  produces  the 
occipital  lobe. 

The  cavity  of  the  vesicle  is  modified  in  form  and  extent  co- 
incidentally  with  the  formation  of  the  corpus  striatum  and 
the  alterations  in  the  ring  lobe.  Just  as  the  ring  lobe  par- 
tially encircles  the  fossa  of  Sylvius,  so  does  the  cavity  of 
the  ventricle  partially  encircle  the  corpus  striatum.  An 
anterior  prolongation  of  the  cavity  extends  into  the  com- 
pleted frontal  lobe  as  the  anterior  cornu  of  the  ventricle,  and 
an  extension  downward  and  forward  into  the  apex  of  the 
.temporal  lobe  constitutes  the  descending  cornu,  while  the 
posterior  horn  is  gradually  protruded  into  the  occipital  lobe  as 
the  latter  develops.  From  the  earliest  stage,  therefore,  until 
the  completed  condition  is  attained,  the  cavity  of  the  ventri- 
cle conforms  in  a  general  way  to  the  shape  of  the  hemi- 
sphere. The  apertures  of  communication  between  the  vesi- 
cles of  the  cerebrum  and  the  cavity  of  the  inter-brain  are  the 
later  Y-shaped  foramen  commune  anterius  or  the  foramen  of 
Monro. 

The  mesial  surfaces  of  the  hemispheres  are  much  modified 
in  character  by  the  development  here  of  two  total  fissures, 
the  arcuate  fissure  and  the  choroid  fissure.  These  appear  in 
the  fifth  week  while  the  vesicles  are  still  separate  from  each 
other  down  to  their  stalks  of  attachment  to  the  inter-brain, 
prior  to  the  development,  therefore,  of  the  corpus  callosum 
and  the  fornix.  The  two  fissures  lie  close  together,  parallel 
with  each  other  and  with  the  margin  of  the  ring  lobe,  their 
course  conforming  to  that  of  the  cavity  of  the  ventricle.  Be- 
ginning near  the  anterior  extremity  of  the  brain,  above  the 


METAMORPHOSIS   OF  THE  FORE-BRAIN   VESICLE.     283 

level  of  the  corpus  striatum,  they  pass  backward  and  then 
downward  and  afterward  forward  to  terminate  near  the  an- 
terior extremity  of  the  temporal  lobe,  thus  incompletely  en- 
circling the  striate  body. 

The  arcuate  fissure  is  the  more  peripherally  placed  of  the 
two.  Its  anterior  portion  lies  just  above  the  region  through- 
out which  adhesions  subsequently  develop  between  the  two 
hemispheres,  or  in  other  words,  above  the  position  of  the 
future  corpus  callosum  (Fig.  138,  a./.).    This  part  of  the  arcu- 


^.f 


Fig.  138.— Mesial  surface  of  left  fore-brain  vesicle  of  brain  shown  in  Fig.  132  ( Fb) : 
f.M,  foramen  of  Monro,  or  opening  into  inter-brain;  a.f,  arcuate  fissure  :  ch.f,  cho- 
roid fissure  ;  r, "  randbogen,"  corresponding  to  future  corpus  callosum  and  fornix ; 
olf,  olfactory  lobe. 

ate  fissure  is  the  sulcus  of  the  corpus  callosum  of  the  mature 
brain.  The  posterior  segment,  that  which  belongs  to  the 
temporal  lobe  (not  present  at  this  stage),  is  the  future  hippo- 
campal  or  dentate  fissure.  The  hippocampal  fissure  is  repre- 
sented upon  the  mesial  wall  of  the  descending  horn  of  the 
lateral  ventricle  by  the  prominence  known  as  the  hippocampus 
major. 

The  choroid  fissure  or  fissure  of  the  choroid  plexus,  forming 
an  incomplete  ring  within,  and  parallel  with,  that  described 
by  the  arcuate  fissure,  encircles  the  corpus  striatum  more 
closely  (Figs.  138,  139).  It  begins  at  the  foramen  of  Monro, 
and  its  anterior  part  lies  under  the  position  of  the  body  of 
the  future  fornix.  It  then  sweeps  around  into  the  temporal 
lobe  and  terminates  near  the  anterior  part  of  the  latter.  The 
fissure  of  the  choroid  plexus,  like  other  total  fissures,  is  an 
infolding  of  the  wall  of  the  cerebral  vesicle.  It  presents  the 
peculiarity,  however,  that  the  infolded  part  of  the  wall  is 
extremely  thin,  consisting  of  but  a  single  layer  of  epithelial 


284  TEXT-BOOK  OF  EMBRYOLOGY. 

cells.  The  pia  mater,  which  everywhere  closely  invests  the 
surface  of  the  brain,  is  infolded  with  the  vesicle-wall,  the  in- 
folded part  becoming  very  vascular  and  constituting  the 
choroid  plexus  of  the  lateral  ventricle.  The  choroid  plexus, 
although  within  the  limits  of  the  ventricle,  is  excluded, 
strictly  speaking,  from  its  cavity  by  the  layer  of  epithelium 
which  still  covers  it  and  which  has  been  simply  pushed  before 
it  into  that  cavity.  Since  the  epithelial  layer  is  very  thin 
and  easily  ruptured,  the  choroid  fissure  is  apparently  an 
opening  into  the  cavity  of  the  ventricle  through  which  the 
pia  enters  ;  in  the  adult  it  is  called  the  great  transverse  fissure 
of  the  brain. 

The  calcarine  fissure,  another  of  the  total  fissures,  develops 
in  the  latter  part  of  the  third  month  as  a  branch  of  the 
arcuate  fissure.  It  bulges  into  the  mesial  wall  of  the  poste- 
rior horn  of  the  ventricle,  producing  the  elevation  known  as 
the  calcar  avis  or  hippocampus  minor.  Since  the  posterior 
horn  of  the  ventricle  is  developed  as  an  extension  of  the  cav- 
ity into  the  backward  prolongation  of  the  ring  lobe  which 
forms  the  occipital  lobe,  the  calcarine  fissure  necessarily  is 
later  in  appearing  than  the  fissures  above  described. 

The  parieto-occipital  fissure  is  added  in  the  fourth  month 
as  a  branch  of  the  calcarine,  effecting  the  definite  demarca- 
tion between  the  parietal  and  occipital  lobes. 

The  fissure  of  Rolando  develops  in  the  latter  part  of  the 
fifth  month  in  two  parts.  The  two  furrows  are  at  first 
entirely  separated  from  each  other  by  an  intervening  area  of 
cortex.  Subsequently  this  part  of  the  cortex  sinks  be- 
neath the  surface,  as  it  were,  since  it  expands  less  rapidly 
than  the  adjacent  regions,  and  in  this  way  the  upper  and 
lower  limbs  of  the  fissure  become  continuous.  The  sunken 
cortical  area  is  to  be  found  even  in  the  adult  brain  as  a  deep 
annectant  gyrus  embedded  in  the  Rolandic  fissure  at  the  po- 
sition of  its  superior  genu.  The  development  of  the  fissure 
of  Rolando  effects  the  division  between  the  frontal  and  pari- 
etal lobes. 

The  collateral  fissure  appears  in  the  sixth  month  as  a 
longitudinal  infolding  of  the  mesial  wall  of  the  hemisphere 


METAMORPHOSIS  OF  THE  FORE-BRAIN    VESICLE.     285 

below  and  parallel  with  the  hippocarapal  fissure.  Being  a 
total  fissure,  its  presence  affects  the  wall  of  the  cavity  of  the 
vesicle,  producing  the  eminentia  coUateralis.  At  about  the 
same  time  the  calloso-marginal  fissure  makes  its  appearance, 
and  this  is  morphologically  continuous,  through  the  medium 
of  the  post-limbic  sulcus,  with  the  collateral  fissure  (Fig.  141). 
These  three  fissures  constitute  the  peripheral  boundary  of  a 
region  of  the  mesial  wall  which  is  known  in  morphology 
as  the  falciform  or  limbic  lobe. 

The  longitudinal  fissure  in  the  early  stage  of  the  growth  of 
the  cerebrum  separates  the  two  vesicles  from  each  other  ex- 
cept at  the  place  where  they  are  attached  to  the  inter-brain ; 
here  the  two  sacs  are  united  by  that  part  of  their  common 
anterior  wall  which  is  immediately  in  front  of  the  apertures 
of  communication  with  the  inter-brain  and  which  is  called 
the  lamina  terminalis. 

The  development  of  adhesions  between  the  mesial  surfaces 
of  the  hemisphere  vesicles  throughout  certain  definite  areas 
marks  the  beginning  of  the  corpus  callosum  and  the  fornix. 
The  fusion  of  these  areas  begins  in  the  third  month  in  the 
region  corresponding  to  the  anterior  pillars  of  the  fornix,  the 
septum  lucidura  and  the  genu  of  the  corpus  callosum ;  in 
the  fifth  and  sixth  months  adhesion  occurs  in  the  position  of 
the  body  of  the  fornix  and  of  the  body  and  splenium  of  the 
corpus  callosum. 

Although  the  central  white  medullary  matter  of  the  cere- 
bral hemisphere  is  covered  almost  universally  by  the  cortical 
gray  matter,  there  is  a  limited  area  of  the  mesial  surface  from 
which  the  gray  matter  is  absent,  leaving  the  white  matter 
exposed.  The  area  of  uncovered  white  matter  has  the  form 
of  a  narrow  band,  which  begins  at  the  base  of  the  hemisphere, 
in  front  of  the  opening  into  the  inter-brain,  extends  upward 
along  the  anterior  wall  of  the  inter-brain,  then  passes  back- 
ward along  its  roof  and  curves  downward  and  outward  behind, 
and  then  forward  under  it,  to  terminate  at  the  front  part  of 
the  temporal  lobe.  Thus  this  white  band,  which  is  known  as 
the  fimbria,  and  which  represents  the  lower  mesial  edge  of 
the  hemisphere,  almost  encircles  the  inter-brain.  The  fimbria 


286 


TEXT-BOOK  OF  EMBRYOLOGY. 


runs  between  the  arcuate  fissure  and  the  fissure  of  the  choroid 
plexus  (Fig.  139,/).    It  holds  such  a  close  relation  to  the  lat- 


FiG.  139.— Mesial  surface  of  left  hemisphere,  brain  of  fetus  of  three  months 
(enlarged) :  /.,  fornix ;  c.c,  beginning  of  corpus  callosum ;  est,  part  of  corpus  stri- 
atum arching  around  fossa  of  Sylvius  ;  a,/.,  anterior,  and  a.f.p.,  posterior  parts  of 
arcuate  fissure ;  ch.J.,  choroid  fissure,  the  concavity  between  which  and  the  corpus 
striatum  accommodates  the  inter-brain,  which  has  been  removed.  The  fissure  is 
occupied  by  the  pia  mater. 

ter  fissure,  being  placed  on  its  peripheral  side,  that  it  consti- 
tutes the  edge  of  the  apparent  opening  into  the  cavity  of  the 
vesicle  through  which  the  pia  mater,  bearing  blood-vessels,  is 
reflected  into  the  interior,  and  Mdiich,  as  pointed  out  above, 
is  the  transverse  fissure  of  the  brain.  The  opening  is  only 
apparent,  however,  since  the  wall  is  still  unbroken,  although 
reduced  to  a  single  layer  of  epithelium.  The  pia  mater,  form- 
ing, with  its  blood-vessels,  the  choroid  plexus  of  the  lateral 
ventricle,  pushes  the  layer  of  epithelium  before  it,  and  al- 
though the  plexus  is  said  to  be  within  the  cavity  of  the  ven- 
tricle, it  is  still  covered  by  the  layer  of  epithelium,  the  epen- 
dyma,  which  lines  that  cavity. 

The  part  of  the  fimbria  that  immediately  overlies  the  roof 
of  the  inter-brain  becomes  intimately  united,  as  noted  above, 
with  the  corresponding  part  of  the  fimbria  of  the  other  hemi- 
sphere, these  fused  portions  of  the  two  fimbriae  forming  a  flat 
triangular  sheet,  the  body  of  the  fornix.  The  anterior  and 
posterior  portions  of  the  fimbria,  which  diverge  from  the 
median  plane,  represent  respectively  the  anterior  and  poste- 
rior limbs  of  the  fornix. 

Noting  the  relation  of  the  anterior  part  of  the  fimbria  to 
the  aperture  of  communication  between  the  inter-brain  and 
the  cerebral  vesicles,  it  becomes  apparent  that  the  anterior 
pillar  of  the  fornix  forms  the  anterior  and  upper  boundary  of 


METAMORPHOSIS  OF  THE  FORE-BRAIN    VESICLE.     287 

the  foramen  of  Monro.  When,  further,  one  considers  the 
relation  of  the  fimbria  to  the  apparent  opening  into  the  ven- 
tricle, through  which  the  pia  mater  is  invaginated  (the  trans- 
verse fissure),  it  is  explained  why  the  edge  of  the  fornix 
appears  as  a  narrow  white  band,  not  only  as  viewed  from 
within  the  ventricular  cavity,  but  also  in  a  mesial  section  of 
the  brain  (Fig.  140,  C). 


Sy  tine  calc 

Fig.  140.— Fetal  brain  at  the  beginning  of  the  eighth  month  (Mihalkovics)  : 
A,  superior,  B,  lateral,  C,  mesial  surface  :  J?,  fissure  of  Rolando ;  •pre,  precentral 
fissure  ;  Sy,  Sylvian  fissure ;  intp,  interparietal  fissure ;  poc,  parieto-oceipital  fissure ; 
pH,  parallel  fissure ;  callm,  callosomarginal  fissure ;  unc,  uncus ;  calc,  calcarine 
fissure. 


Another  important  region  of  fusion  of  the  opposed  mesial 
surfaces  of  the  hemispheres  is  that  corresponding  to  the 
future  corpus  callosum.  Throughout  this  area  the  hemispheres 
closely  unite  with  each  other.  The  line  of  fusion  begins 
at  the  bases  of  the  vesicles,  some  little  distance  in  front 
of  the  anterior  parts  of  the  fimbriae  (Fig.  139,  c.c),  and  after 
passing  upward  and  forward,  curves  horizontally  backward 


288  TEXT-BOOK  OF  EMBRYOLOGY. 

ill  close  relation  with  the  fused  portions  of  the  fimbriae,  now 
the  body  of  the  fornix.  The  adhesion  begins  at  the  anterior 
part  in  the  third  month,  and  aifects  the  region  of  the  body 
and  spleniura  of  the  future  corpus  callosum  in  the  fifth  and 
sixth  months.  Fibers  j)enetrate  from  one  hemisphere  to  the 
other  throughout  this  zone  of  contact,  intimately  uniting  the 
cerebral  hemispheres.  The  corpus  callosum  is  therefore  a 
great  commissure  between  the  two  halves  of  the  cerebrum, 
and  is  necessarily  composed  of  fibers  having  a  transverse 
direction. 

While  the  back  part  of  the  corpus  callosum  lies  over  the 
body  of  the  fornix  and  is  in  close  contact  with  it,  the  front 
part  of  the  body  of  the  corpus  collusum,  as  also  its  genu  or 
curve  and  its  rostrum  or  ascending  part  are  at  some  distance 
from  the  front  parts  of  the  fimbriae.  In  other  words,  while  the 
great  longitudinal  fissure  extends  at  first  to  the  bases  of  the 
cerebral  vesicles,  this  fissure  is  made  relatively  less  deep  by 
the  adhesions  which  occur  between  the  mesial  walls  and  which 
result  in  the  development  of  the  corpus  callosum  ;  and  the 
space  below  the  anterior  part  of  the  corpus  callosum,  between 
it  and  the  anterior  parts  of  the  fimbriae  (Fig.  140,  C),  is  an 
isolated  part  of  the  great  longitudinal  fissure.  This  space  is 
bounded  on  either  side  by  that  part  of  the  wall  of  the  corres- 
ponding cerebral  vesicle  or  hemisphere  which  is  limited  above 
and  in  front  by  the  corpus  collusum,  and  behind  by  the  ante- 
rior part  of  the  fimbria  or  anterior  limb  of  the  fornix.  The 
space  is  the  so-called  fifth  ventricle  of  the  adult  brain.  The 
circumscribed  parts  of  the  mesial  walls  of  the  hemispheres, 
which  form  the  lateral  walls  of  the  space,  together  constitute 
the  septum  lucidum.  The  parts  of  the  hemisphere  walls  that 
become  the  septum  lucidum  do  not  participate  in  the  process 
of  fusion  mentioned  above.  Their  surfaces  are  in  contact, 
however,  and  do  not  develop  the  typical  gray  cortical  matter, 
such  as  appears  elsewhere  upon  the  surface  of  the  cerebrum. 
Cortical  gray  matter  is  produced  here,  but  only  in  rudi- 
mentary form. 

From  what  has  been  said,  it  will  be  seen  that  the  two 
layers  of  the  septum  lucidum  are  circumscribed  and  opposed 


METAMORPHOSIS  OF  THE  FORE-BRAIN   VESICLE.     289 


parts  of  the  mesial  walls  of  the  hemispheres ;  that  the  fifth 
ventricle  is  not  a  true  ventricle  but  an  isolated  part  of  the 
longitudinal  fissure  having  no  connection  whatever  with  the 
system  of  ventricular  cavities ;  and  that  this  so-called  ventri- 
cle is  not,  like  the  true  ventricles  of  the  brain,  lined  with 
ependyma,  but  with  atrophic  gray  cortical  matter. 

The  limbic  lobe  has  been  referred  to  as  that  part  of  the 
mesial  surface  of  the  hemisphere  which  is  circumscribed  by 
the  calloso- marginal  fissure,  the  post-limbic  sulcus,  and  the 
collateral  fissure.  It  is  limited  centrally  by  the  fissure  of  the 
corpus  callosum  and  the  hippocampal  fissure,  which  are 
represented  in  the  fetal  brain  by  the  single  uninterrupted 
arcuate  fissure.  Hence  the  limbic  lobe  would  include  the 
gyrus  fornicatus,  the  isthmus,  and  the  gyrus  uncinatus,  which 
constitute  morphologically  a  single  ring-like  convolution. 
Schwalbe,  however,  includes  with  this  so-called  limbic  lobe 
all  the  surface  of  the  mesial  wall  of  the  hemisphere  included 
between  the  arcuate  fissure  and  the  fissure  of  the  choroid 
plexus  (Fig.  138),  designating  it  the  falciform  lobe  (Fig.  141). 


roynix 


Gyrus  Su^ra-caUosui 


Corpus  CaUasiLW 


TySct 
GyriAS  SuicaUostiS^ 

foramert-c^Jl^onrO 
JUicajtrS 
^     1     .  f'laura.MtppacajTu 
tiniriiL.  ' 

,       Fig.  141.— Diagram  of  the  limbic  lobe  (after  Quaiu). 

The  falciform  lobe  therefore  consists  of  two  ring-like  convo- 
lutions, one  within  the  other,  the  two  being  separated  from 
each  other  by  the  arcuate  fissure  (the  adult  callosal  and  den- 
tate fissures)  and  being  limited  centrally  by  the  fissure  of  the 
choroid    plexus  (the  great  transverse   fissure  of  the  adult 

19 


290  TEXT-BOOK  OF  EMBRYOLOGY. 

brain).  While  the  outer  of  these  concentric  convolutions — 
the  limbic  lobe  of  Broca — develops  into  the  fornicate  or  cal- 
losal,  the  isthmian,  and  the  uncinate  gyri,  the  inner  ring 
differentiates  but  slightly,  its  cortical  matter  remaining 
atrophic.  The  atrophic  condition  of  the  cortex  here  is  asso- 
ciated with  those  adhesions  between  the  mesial  walls  of  the 
hemispheres  that  result  in  the  formation  of  the  corpus  cal- 
losum  and  the  septum  lucidura.  By  these  adhesions  the 
continuity  of  the  inner  concentric  convolution  is  broken,  and 
it  is  therefore  represented,  after  the  development  of  the  corpus 
callosum,  by  the  atrophic  gray  matter  of  the  septum  lucidum, 
by  the  gyrus  dentatus,  and  by  the  lateral  longitudinal  striae 
on  the  free  surface  of  the  corpus  callosum,  the  latter  being  an 
atrophic  or  rudimentary  convolution.  Since  the  transverse 
fissure  of  the  brain  is  the  centric  boundary  of  the  ring,  the 
fornix  is  also  a  part  of  the  falciform  lobe.  To  sum  up,  the 
falciform  lobe  includes  the  gyrus  fornicatus,  the  isthmus,  the 
gyrus  uncinatus,  the  lateral  longitudinal  strise  or  taenia  tectse 
of  the  corpus  callosum,  the  gyrus  dentatus,  the  laminse  of  the 
septum  lucidum  and  the  fornix. 

The  olfactory  lobe  or  rhinencephalon  is  an  outgrowth  from 
the  vesicle  of  the  cerebral  hemisphere.  Its  development  be- 
gins in  the  fifth  week  by  the  pouching-out  of  the  wall  of  the 
vesicle  near  the  anterior  part  of  its  floor  (Figs.  131  and  133). 
This  diverticulum,  which  contains  a  cavity  continuous  with 
that  of  the  vesicle,  grows  forward  and  soon  becomes  some- 
what club-shaped.  In  the  selachians  (sharks  and  dog-fish) 
the  projection  attains  a  great  relative  size,  the  olfactory  lobes 
in  these  animals  being  one  of  the  most  conspicuous  parts  of 
the  brain.  In  all  mammals  except  man  it  is  well  developed, 
and  in  the  horse  its  cavity  persists  throughout  life.  In  man 
the  cavity  soon  becomes  obliterated  and  the  lobe  itself  in 
part  aborts.  The  protruded  portion,  becoming  more  dis- 
tinctly club-shaped,  differentiates  into  the  olfactory  bulb  and 
the  olfactory  tract,  the  position  of  the  original  cavity  being 
indicated  by  a  more  or  less  central  mass  of  neuroglia  con- 
spicuous in  cross-sections  of  those  structures.  The  proximal 
portion  of  the  olfactory   lobe  is   represented  in   the  adult 


METAMORPHOSIS  OF  THE  FORE-BRAIN  VESICLE.     291 

human  brain  by  the  gray  matter  of  the  anterior  perforated 
lamina  (or  space),  and  by  the  trigonum  olfactorium  and  the 
area  of  Broca,  as  well  as  by  the  inner  and  outer  roots  of  the 
olfactory  tract  (note  olfactory  lobe  of  dog's  brain,  Fig.  142). 


Fig.  142.— Base  of  dog's  brain :  oL,  olfactory  lobe ;  a.p.s.,  region  corresponding  to 

anterior  perforated  space,  which  is  included  in  the  olfactory  lobe ;  f.S.,  fissure  of 
Sylvius;  g.h.,  hippocampal  gyrus,  developed  to  a  greater  degree  than  in  human 
brain;  s.,  sectional  surface  of  olfactory  lobe;  os.,  olfactory  sulcus. 


Because  of  the  relation  of  the  place  of  evagination  of  the 
olfactory  lobe  to  the  fossa  of  Sylvius,  it  happens  that  a  part 
of  this  lobe,  the  anterior  perforated  lamina,  is  situated  at  the 
commencement  of  the  fissure  of  Sylvius  and  that  it  is  in  con- 
tinuity with  both  extremities  of  the  ring  lobe ;  hence,  the 
olfactory  lobe  is  connected  with  both  extremities  of  the  falci- 
form lobe.  To  express  it  in  the  language  of  human  anatomy, 
the  outer  or  lateral  root  of  the  olfactory  tract  is  connected 
with  the  gyrus  uncinatus,  while  the  inner  or  mesial  root  may 
be  traced  to  the  fore  part  of  the  gyrus  fomicatus. 

After  what  has  been  said,  the  reader  need  scarcely  be  re- 
minded that  the  olfactory  bulb  and  tract,  often  erroneously 
referred  to  as  the  olfactory  nerve,  are  parts  of  a  lobe  of  the 


292 


TEXT-BOOK  OF  EMBRYOLOGY. 


brain,  a  lobe  which  in  man  is  rudimentary  but  which  in  all 
other  mammals  is  well  developed. 

Tabulated  Resume  of  the  Derivatives  of  the  Brain-vesicles. 


Brain- 
vesicles. 

Floor. 

Roof. 

Lateral  walls. 

Cavity. 

After- 
brain 
vesicle. 

Medulla 
oblongata. 

Tela  choroidea 
inferior. 

Inferior     pedun- 
cles of  cerebel- 
lum. 

Fourth 
■    ven- 
tricle. 

Hind- 
brain 
vesicle. 

Pons  Varolii. 

Posterior  medul- 
lary        velum. 
Cerebellum. 
Anterior      me- 
dullary velum. 

Middle  and  supe- 
rior   peduncles 
of  cerebellum. 

Mid-brain 
vesicle. 

Peduncles  of  cer- 
ebrum.    Poste- 
rior perforated 
space. 

Corpora     quadri- 
gemina.     Lam- 
ina quadrigem- 
ina. 

Brachia.      Inter- 
nal   geniculate 
bodies.- 

Aqueduct 
of  Syl- 
vius. 

Inter- 
brain 
vesicle. 

Corpora    albican- 
tia.     Tuber  ci- 
nereum,  infun- 
dibulum,     and 
part    of    hypo- 
physis.      Optic 
chiasm. 

Pineal  body.  Pos- 
terior   commis- 
sure.      Epithe- 
lium of  velum 
interpositum. 

Optic  thalami. 

Third 
ven- 
tricle. 

Secondary 
fore- 
brain 
vesicle. 

Anterior      perfo- 
rated    lamina. 
Corpus       stria- 
tum.   Island  of 
Reil.    Olfactory 
lobe. 

Convolutions     of     cerebral     hemi- 
spheres.    Corpus  callosum.     For- 
nix.   Septum  lucidum. 

Lateral 
ven- 
tricles. 

THE  DEVELOPMENT  OF  THE  PERIPHERAL  NERVOUS 
SYSTEM. 

The  development  of  the  peripheral  nervous  system  is  still 
involved  in  some  degree  of  obscurity.  In  general  terms  it 
may  be  stated  that  the  peripheral  nervous  apparatus  is  de- 
rived as  an  extension  of  the  central  cerebro-spinal  axis. 

In  the  case  of  the  spinal  nerves,  each  nerve-trunk  is  com- 
posed of  both  motor  and  sensory  fibers,  the  former  being  in 
continuity  with  the  spinal  cord  through  the  medium  of  the 
anterior  or  motor  roots,  and  the  latter  through  the  posterior 
or  sensory  roots,  each  sensory  root  possessing  a  ganglion. 
The  cranial  nerves  exhibit  a  less  regular  composition.  While 
the  trigeminal  nerve,  for  example,  arises  by  two  roots,  after 
the  manner  of  a  spinal  nerve,  some  others  correspond  in  rela- 
tive position  and  in  mode  of  development  to  the  ventral  or 
motor  roots  of  the  spinal  nerves,  and  still  others  are  equiva- 
lent to  the  sensory  spinal  roots. 


ORIGIN  OF  THE  GANGLIA. 


•293 


The  development  of  the  sensory  nerve-fibers  is  dependent 
upon  and  is  preceded  by  that  of  the  ganglia  of  the  posterior 
roots  of  the  spinal  nerves,  and  of  several  ganglia  of  the  head 
region  which  are  related  to  the  development  of  certain  of  the 
cranial  nerves.     Hence  the  consideration  of  the  genesis  of 


Fig.  143.—^,  cross-section  through  an  embryo  of  Pristii;rus  (after  Rabl).  The 
primitive  segm^ents  are  still  connected  with  the  remaining  portion  of  the  middle 
germ-layer.  At  the  region  of  transition  there  is  to  be  seen  an  outfolding  (st)  from 
which  the  skeletogenous  tissue  is  developed ;  ch,  chorda ;  spg,  spinal  ganglion ;  mp, 
muscle-plate  of  the  primitive  segment;  scA,  subchordal  rod;  ao,  aorta;  it,  inner 
germ-layer;  pmb,  parietal,  vmh,  visceral  middle  layer.  B,  cross-section  through  a 
lizard  embryo  (after  Sagemehl)  :  rm,  spinal  cord ;  spg,  lower  thickened  part  of  the 
neural  ridge;  spg',  its  upper  attenuated  part,  which  is  continuous  with  the  roof  of 
the  neural  tube ;  us,  primitive  segment. 

the  ganglia  must  precede  the  account  of  the  growth  of  the 
sensory  nerve-fibers. 

The  origin  of  the  ganglia  is  connected  with  the  early 
history  of  the  evolution  of  the  neural  tube.     Just  after  the 


294-  TEXT-BOOK  OF  EMBRYOLOGY. 

sides  of  the  medullary  plate  {vide  p.  255)  have  united  with 
each  other  to  form  the  neural  tube,  there  appear  two  ridges 
of  cells  between  the  tube  and  the  epidermis,  one  on  each  side 
of  the  raphe  or  line  of  union  of  the  sides  of  the  tube.  These 
ridges  are  the  neural  crests  (Fig.  143).  They  first  appear  in 
the  region  of  the  hind-brain  and  advance  from  this  point 
both  headward  and  tailward.  The  ganglia  develop  from 
these  neural  crests.  The  cells  of  the  neural  crest  are  usually 
described  as  growing  out  from  the  neural  tube,  though  ac- 
cording to  His  it  is  probable  that  they  originate  singly  from 
the  ectoderm. 

The  mass  of  cells  composing  the  neural  crest  grows  out- 
ward and  then  ventrad  along  the  wall  of  the  neural  tube,  and 
very  soon  undergoes  segmentation  into  a  number  of  cell- 
masses  which  are  the  rudimentary  ganglia.  In  the  spinal 
region  the  number  of  segments  corresponds  to  the  num- 
ber of  future  spinal  nerves.  In  the  head  region  there  are 
four  segments.  These  latter,  the  cephalic  ganglia,  will  be 
referred  to  subsequently. 

The  segmentation  of  the  neural  crest  corresponds  in  the 
main  with  the  segmentation  of  the  paraxial  plate  of  the 
mesoderm,  whereby  the  myotomes  are  produced,  and  each 
segment  lies  upon  the  inner  side  of  a  myotome.  The  con- 
nection of  the  segments  with  the  neural  tube  becomes  re- 
duced in  each  case  to  a  slender  strand,  the  point  of  continuity 
of  which  with  the  tube  is  shifted  farther  away  from  the 
median  line,  as  development  progresses,  to  correspond  with 
the  dorsolateral  position  of  the  sensory  nerve-roots  in  the 
mature  condition. 

The  cells  of  the  ganglia  acquire  axis-cylinder  processes, 
each  such  process,  upon  further  elongation,  becoming  the 
axis  cylinder  of  a  future  nerve-fiber.  The  cells  being  bi- 
polar, each  one  gives  origin  to  two  nerve-fibers,  one  passing 
to  the  spinal  cord  and  one  going  to  the  periphery  of  the 
body  as  a  sensory  nerve-fiber.  Thus  the  ganglia  are  made 
up  of  cells  which  are  interpolated  in  the  course  of  the  sen- 
sory nerve-fibers,  and  these  cells  may  be  regarded  as  having 
migrated  from  the  developing  cerebrospinal  axis,  or,  if  the 


ENVELOPES  OF  THE  NEBVE-FIBER.  295 

view  of  His  be  accepted,  from  the  region  of  the  ectoderm 
from  which  the  tube  originates,  their  connection  with  the 
axis  being  maintained  by  the  gradually  lengthening  out  axis- 
cylinder  process. 

The  development  of  the  motor  nerve-fibers  diflPers  from 
that  of  the  sensory.  These  fibers,  or  at  least,  the  axis  cylin- 
ders of  the  fibers,  are  the  elongated  neurits  of  nerve-cells  of 
the  spinal  cord  and  brain.  The  neuroblasts  of  the  thickened 
neural  tube,  as  they  become  fully  difi'erentiated  nerve-cells, 
migrate  from  their  central  position  into  the  mantel  layer,  or 
superficial  stratum  (Fig.  124).  On  the  distal  side  of  the  nu- 
cleus of  the  cell,  the  protoplasm  first  becomes  massed  and 
then  lengthens  out  to  form  an  axis-cylinder  process  or  neurit, 
which  in  all  vertebrate  animals  grows  out  from  the  cerebro- 
spinal axis  to  form  a  motor  nerve-fiber. 

Although,  in  the  case  of  the  spinal  nerves,  the  motor  and 
sensory  fibers  are  separated  from  each  other  at  their  origin 
from  the  cord,  they  soon  intermingle  to  constitute  a  spinal 
nerve-trunk.  In  certain  lower  types,  as  cyclostomes  and 
amphioxus,  the  motor  and  the  sensory  fibers  permanently 
pursue  separate  routes  to  their  peripheral  distribution. 

The  envelopes  of  the  nerve-fiber  are  acquired  at  a  rela- 
tively late  period.  The  appearance  of  the  neurilemma  pre- 
cedes that  of  the  white  substance  of  Schwann.  Both  these 
investments  are  derived  from  the  mesoderm.  The  cells  of 
the  latter  apply  themselves  to  the  nerve  and,  penetrating 
between  the  fibers,  become  arranged  as  an  enveloping  layer 
upon  each  axis  cylinder,  ultimately  forming  a  complete 
sheath,  the  neurilemma.  The  persistent  nuclei  of  these  cells, 
scantily  surrounded  Avith  protoplasm,  constitute  the  nerve- 
corpuscles  of  the  neurilemma.  The  medulla,  or  white  sub- 
stance of  Schwann,  is  formed  at  a  considerably  later  period 
within  the  neurilemma.  The  deposit  of  the  medullary  sheath 
varies  as  to  time  for  different  groups  of  fibers — although  the 
time  is  constant  for  each  group — and  proceeds  always  in  a 
direction  away  from  the  cell  from  which  the  fiber  originates, 
or,  differently  expressed,  in  the  direction  in  which  the  fiber 
conveys  impulses.    Thus,  in  the  spinal  cord,  groups  of  afferent 


296  TEXT-BOOK  OF  EMBRYOLOGY. 

fibers  may  be  distinguished  from  those  that  are  efferent  by 
observing  the  direction  in  which  the  medullary  sheath  devel- 
ops—that is,  whether  the  sheath  appears  first  at  the  upper  end 
of  the  fiber  or  at  the  lower  end. 

The  cranial  nerve-fibers  in  their  development  follow  in  the 
main  the  same  general  principles  that  govern  the  growth  of 
the  spinal  nerves.  That  is  to  say,  the  motor  fibers  grow  out 
as  extensions  of  the  axis-cylinder  processes  of  nerve-cells  of 
the  cephalic  part  of  the  neural  tube  and  the  sensory  fibers 
proceed  from  the  cells  of  outlying  ganglia,  or  in  tlie  case  of 
at  least  one  nerve,  the  olfactory,  from  infolded  and  highly 
specialized  cells  of  the  ectoderm. 

The  cephalic  ganglia,  four  in  number,  have  been  referred 
to  as  resulting  from  the  segmentation  of  the  head-region  of 
the  neural  crest.  As  previously  stated,  the  neural  crest 
begins  to  grow  first  in  the  region  of  the  hind-brain  and 
extends  from  this  point  both  forward  and  backward,  occupy- 
ing a  position  upon  the  roof  or  dorsal  wall  of  the  hind-brain. 
The  part  of  the  neural  crest  belonging  to  the  head-region 
then  divides  into  the  four  masses  or  head-ganglia  which  are 
designated  respectively  the  first  or  trigeminal,  the  second  or 
acusticofacial,  the  third  or  glossopharyngeal,  and  the  fourth 
or  vagal,  ganglia. 

The  trigeminal  ganglion,  which  is  very  large,  becomes  di- 
•vided  into  a  smaller  anterior  portion,  the  ophthalmic  or  ciliary 
ganglion,  and  a  larger  posterior  segment,  the  trigeminal 
ganglion  proper.  These  two  become  widely  separated  during 
the  progress  of  development,  since  they  constitute  respec- 
tively the  later  ciliary  and  Gasserian  ganglia,  the  ciliary 
ganglion  belonging  to  the  ophthalmic  division  of  the  fifth 
nerve,  while  the  trigeminal  belongs  to  the  superior  maxillary 
division  and  the  sensory  part  of  the  inferior  maxillary  divi- 
sion of  the  fifth.  Their  nerve-cells  give  rise  to  the  sensory 
fibers  of  these  trunks  in  the  same  manner  that  the  cells  of 
the  spinal  ganglia  produce  the  sensory  fibers  of  the  spinal 
nerves.  By  some  observers,  the  sphenopalatine  (Meckel's) 
ganglion  and  the  otic,  and  possibly  the  submaxillary  ganglia, 
are  to  be  regarded  as  offshoots  from  the  Gasserian  ganglion. 


'  FOURTH  CEPHALIC  GANGLION.  297 

The  acusticofacial  ganglion,  after  its  migration  from  its 
original  position  on  the  dorsum  of  the  hind-brain,  lies  just 
in  front  of  the  otic  vesicle.  This  ganglion  subsequently 
divides  into  the  facial  and  the  acoustic  ganglia.  The  facial 
ganglion,  the  geniculate  ganglion  or  intumescentia  ganglio- 
formis  of  the  facial  nerve,  situated  in  the  facial  canal  of 
the  temporal  bone,  although  described  as  a  ganglion  upon 
a  motor  nerve,  the  facial,  is,  in  reality,  connected  mainly 
with  the  pars  intermedia,  a  bundle  of  sensory  fibers  issuing 
from  the  nucleus  of  origin  of  the  glossopharyngeal  nerve. 
It  is  equivalent  therefore  to  a  spinal  ganglion. 

The  acoustic  portion  of  the  acusticofacial  ganglion  divides 
still  further  to  become  the  ganglion  on  the  vestibular  part  of 
the  auditory  nerve,  and  the  ganglion  spirale  of  the  cochlear 
division  of  the  auditory,  which  latter  is  situated  in  the  spiral 
canal  of  the  modiolus.  It  is  considered  probable  that  the 
lateral  accessory  auditory  nucleus,  which  is  connected  with 
the  cochlear  fibers  of  the  auditory  nerve  and  lies  on  the  outer 
side  of  the  restiform  body,  is  also  a  part  of  the  acoustic 
ganglion.  From  the  cells  of  the  vestibular  ganglion,  which 
is  situated  in  the  internal  meatus,  centrifugal  fibers  develop 
to  form  the  vestibular  nerve,  while  other  centripetally  growing 
fibers  become  the  ventral  or  mesial  (vestibular)  root  of  the 
auditory  nerve.  The  cochlear  ganglion  in  the  same  way  gives 
rise  to  the  cochlear  branch  of  the  nerve  and  to  its  dorsal  or 
lateral  root.  Thus  the  auditory  nerve  and  its  ganglia  corre- 
spond respectively  to  the  sensory  root  of  a  spinal  nerve  and 
to  a  spinal  ganglion. 

The  third  cephalic  ganglion  becomes  the  ganglion  of  the 
glossopharyngeal  nerve,  undergoing  segmentation  to  form 
the  upper  or  jugular  and  the  lower  or  petrous  ganglia  of  this 
nerve,  while  the  axis-cylinder  processes  of  its  cells  lengthen 
out  to  become  the  sensory  fibers. 

The  fourth  cephalic  ganglion  similarly  becomes  the  two 
ganglia  of  the  pneumogastric  nerve  and  gives  rise  to  its 
sensory  fibers. 

While  the  motor  fibers  of  the  cranial  nerves  develop  by 
the  outgrowth  of  the  axis-cylinder  processes  of  the  motor 


298  TEXT-BOOK  OF  EMBRYOLOGY. 

nerve-cells  of  the  brain,  and  thus  correspond  in  manner  of 
development  with  the  spinal  motor  fibers,  there  is  a  modifica- 
tion as  to  their  point  of  emergence  from  the  central  axis. 
Instead  of  issuing  in  line  with  the  spinal  motor  roots,  there 
are  two  sets  of  cranial  motor  roots,  the  ventral  and  the 
lateral.  Both  arise  from  the  cells  of  the  ventral  zone  of  the 
neural  tube  and  thus  correspond  in  point  of  origin  with  the 
spinal  motor  fibers,  but  the  cells  from  which  proceed  the 
fibers  of  the  ventral  roots  are  situated  in  the  ventral  part  of 
this  zone,  whereas  the  parent  cells  of  the  lateral  roots  lie 
near  its  dorsal  edge,  close  to  the  deep  connections  of  the 
sensory  fibers.  It  happens  therefore  that  the  lateral  roots 
emerge  in  close  proximity  to  the  dorsal  or  sensory  bundles, 
the  two  apparently  constituting  one  nerve-trunk.  The  motor 
fibers  of  the  fifth,  seventh,  ninth,  and  tenth  nerves  have  this 
lateral  position,  and  are  so  closely  identified  with  the  sensory 
fibers  that  the  two  sets  form  one  trunk  in  each  case. 

The  ventral  motor  nerve-roots  emerge  in  line  with  the 
ventral  or  motor  roots  of  the  spinal  nerves.  The  only  cranial 
nerves  which  represent  persistent  ventral  motor  roots  are  the 
abducens  and  the  hypoglossal. 

A  still  further  modification  in  the  cranial  nerves  is  ])resented 
by  their  relation  to  the  segmentation  of  the  head.  As  pointed 
out  above,  the  segmentation  of  the  spinal  part  of  the  neural 
crest  is  in  correspondence  with  the  segmentation  of  the  trunk, 
and  each  spinal  nerve  therefore  may  be  regarded  as  belonging 
to  a  particular  segment  of  the  trunk.  In  the  case  of  the  cra- 
nial nerves,  however,  there  is  no  such  regular  correspondence, 
since  in  some  instances,  several  nerves  are  referred  to  one 
head-segment,  while  in  others,  one  nerve  belongs  to  several 
segments.  An  example  of  the  latter  is  furnished  by  the 
hypoglossal,  which  arises  from  the  side  of  the  medulla  by  a 
series  of  bundles  of  fibers  which  are  referable  to  several 
segments. 

As  will  be  seen  later,  in  the  account  of  the  development 
of  the  nose  and  of  the  eye,  the  olfactory  and  optic  nerves 
exhibit  certain  peculiarities  which  set  them  apart  from  the 
other  cranial  nerves. 


VENTRAL  MOTOR  NERVE-ROOTS.  299 

From  what  has  been  said,  it  will  be  apparent  that  the 
cranial  nerves  develop  in  a  far  less  regular  manner  than  the 
spinal  nerves,  and  that  consequently  their  trunks  consist  in 
some  cases  of  only  sensory  fibers,  in  other  cases  of  only 
motor  fibers,  and  in  still  others,  of  both  varieties.  Typically, 
each  cranial  nerve  would  have  a  dorsal  sensory  root  with  a 
ganglion,  and  two  motor  roots,  one  lateral  and  the  other  ven- 
tral. But  by  the  suppression  of  one  or  two  of  these  typical 
roots  there  will  be  produced  a  nerve,  for  example,  represent- 
ing only  the  ventral  root,  as  the  sixth  and  twelfth  nerves,  or 
a  trunk  containing  sensory  and  lateral  motor  fibers,  as  the 
vagus,  or  a  nerve  consisting  solely  of  sensory  fibers,  as  the 
auditory. 

By  way  of  recapitulation  the  cranial  nerves  may  be  briefly 
considered  seriatim  : 

First  Pair. — The  olfactory  nerve-filaments  grow  centri- 
petally  from  the  olfactory  epithelium  of  the  nasal  mucous 
membrane. 

Second  Pair. — The  optic  nerve  is  not  a  true  nerve  (see 
Chapter  XVI.). 

Third  Pair. — The  oculomotor  nerve  represents  a  persistent 
lateral  motor  root  of  the  first  head-segment  (the  ophthalmic 
division  of  the  fifth  nerve  being  the  sensory  root  of  the  same 
segment). 

Fourth  Pair. — The  trochlear  nerve  represents  a  lateral 
motor  root  and  belongs  to  the  second  head-segment. 

Fifth  Pair. — The  trifacial  or  trigeminal  nerve,  containing 
sensory  and  motor  fibers,  represents  a  persistent  lateral  motor 
root  and  a  dorsal  sensory  root.  The  ophthalmic  portion  of 
the  sensory  root  belongs  to  the  first  head-segment,  while  all 
the  remaining  fibers,  with  the  fourth  nerve,  are  assigned  to 
the  second  segment. 

Sixth  Pair. — The  abducens  develops  as  a  ventral  motor 
root  and  belongs  to  the  third  and  possibly  to  the  fourth 
segments. 

Seventh  and  Eighth  Pairs. — The  acusticofacialis  nerve,  or 
the  facial  and  auditory  nerves,  develop  as  a  single  nerve  with 
several  roots.     The  auditory  nerve  and  the  sensory  fibers  of 


300  TEXT-BOOK  OF  EMBRYOLOGY. 

the  facial — that  is,  the  pars  intermedia — correspond  to  a  dor- 
sal sensory  root,  the  division  of  the  acusticofacial  ganglion  into 
the  several  ganglia  of  the  auditory  nerve  and  the  geniculate 
ganglion  of  the  facial  accounting  for  the  division  of  the  root 
into  the  auditory  trunk  and  the  pars  intermedia.  (The  sen- 
sory fibers  of  the  facial  pass  off  through  the  chorda  tympani 
to  go  to  the  tongue  as  special-sense  fibers.)  The  motor  fibers 
of  the  facial  develop  as  a  lateral  motor  root,  originating 
from  cells  in  the  ventral  zone.  These  two  nerves,  with  the 
sixth,  belong  to  the  third  and  possibly  to  the  fourth  head- 
segments. 

Ninth  Pair. — The  glossopharyngeal  nerve,  made  up  largely 
of  sensory  fibers,  represents  a  dorsal  sensory  root  and  a  lateral 
motor  root,  the  fibers  of  which  latter  grow  out  from  cells  in 
the  dorsal  part  of  the  ventral  zone  of  His,  the  later  nucleus 
ambiguus.     It  belongs  to  the  fifth  head-segment. 

Tenth  Pair. — The  vagus  develops  in  the  same  manner  as 
the  glossopharyngeal. 

Eleventh  Pair. — The  spinal  accessory  represents  in  part 
motor  spinal  roots  and  in  part  probably  the  lateral  motor  and 
dorsal  sensory  roots  of  the  cranial  nerves. 

Twelfth  Pair. — The  hypoglossal  develops  as  the  ventral 
motor  roots  of  several  segments,  being  identical  in  mode  of 
origin  with  the  anterior  roots  of  the  spinal  nerves.  This 
nerve  and  the  vagus  belong  to  the  head-segments  from  the 
sixth  to  the  tenth  inclusive. 

THE  DEVELOPMENT  OF  THE  SYMPATHETIC  SYSTEM. 

There  are  two  views  as  to  the  origin  of  the  sympathetic 
system.  One  theory,  based  upon  the  investigations  of  Pater- 
son,  is  that  the  gangliated  cord  of  the  sympathetic  is  differ- 
entiated from  mesodermic  cells,  the  cell-cord  thus  formed 
acquiring,  secondarily,  connections  with  the  spinal  nerves, 
and  ])resenting  still  later  the  enlargements  which  constitute 
the  ganglia. 

The  more  generally  accepted  view,  based  upon  the  re- 
searches of  Balfour  and  the  later  work  of  Onodi  and  His,  is 
that  the  sympathetic  ganglia  develop  as  offshoots  from  the 


DEVELOPMENT  OF  THE  SYMPATHETIC  SYSTEM.   301 

ventral  extremities  of  the  spinal  ganglia.  Each  little  mass, 
which  has  budded  off  from  a  spinal  ganglion,  moves  some- 
what toward  the  ventral  surface  of  the  body,  its  bond  of 
union  with  the  parent  spinal  ganglion  being  drawn  out  to  a 
slender  cord,  the  representative  of  the  future  ramus  com- 
municans.  Each  primitive  sympathetic  ganglion  sends  out 
two  small  processes,  one  growing  tail  ward  from  its  lower  ex- 
tremity, and  one  in  the  opposite  direction  from  its  upper  end, 
the  approaching  processes  from  each  two  adjacent  ganglia 
meeting  and  uniting  and  thus  secondarily  establishing  the 
connection  between  the  different  ganglia  of  one  side  of  the 
body  and  forming  the  gangliated  cord  of  the  sympathetic. 
From  these  ganglia  migrating  cells  probably  pass  out  to 
develop  into  the  secondary  ganglia  of  certain  viscera,  as  His 
has  shown  to  be  the  mode  of  origin  of  the  ganglia  of  the 
heart. 


CHAPTER   Xyi. 

THE   DEVELOPMENT  OF   THE  SENSE   ORGANS. 

In  the  organs  of  the  senses  we  have  to  do  with  peripheral 
nervous  mechanisms  of  greater  or  less  degrees  of  complexity, 
the  essential  elements  of  which  are  elaborately  modified  or 
specialized  neuro-epithelial  cells.  These  nenro-epithelial 
structures  are  specialized  cells  of  the  ectoderm,  derived  from 
it  either  directly,  by  the  infolding  of  patches  of  ectodermic 
epithelium,  as  in  the  case  of  the  olfactory  cells,  or  indirectly, 
by  growth  outward  from  the  central  nervous  system,  as  in 
the  case  of  the  retina.  The  organs  of  the  sense  of  touch,  the 
tactile  corpuscles  of  the  skin  and  mucous  membranes,  are 
distributed  somewhat  irregularly,  while  such  highly  special- 
ized structures  as  the  organs  of  the  special  senses  of  vision, 
hearing,  smell,  and  taste  are  provided  with  special  protective 
and  accessory  apparatuses. 

THE  DEVELOPMENT  OF  THE  EYE. 

It  will  perhaps  facilitate  the  comprehension  of  the  general 
principles  involved  in  the  development  of  the  eye  if  its 
function  as  the  organ  of  vision  is  kept  in  mind,  and  if, 
therefore,  the  retina  and  the  optic  nerve  are  recognized  as 
the  essential  parts  of  the  organ,  and  the  other  structures  as 
accessories.  The  retina  and  the  optic  nerve  are  an  out- 
growth from  the  brain,  the  rod-  and  cone- visual  cells  of  the 
former  being  epithelial  cells  so  specialized  as  to  serve  as 
percipient  elements,  while  the  optic  nerve-fibers  are  the  con- 
ducting medium.  To  allow  of  the  penetration  and  refraction 
of  the  rays  of  light,  the  overlying  epidermis  differentiates 
into  a  transparent  and  refractive  medium,  the  crystalline 
lens,  and  the  necessary  protection  and  means  of  nourishment 

302 


THE  DEVELOPMENT  OF  THE  EYE. 


303 


are  provided  by  the  other  constituents  of  the  eyeball.  Fur- 
ther protection  is  furnished  by  two  folds  of  modified  skin 
and  subcutaneous  tissue,  the  eyelids,  and  lastly  for  the 
lubrication  and  still  further  protection  of  the  exposed  part 
of  the  eyeball,  there  is  formed  still  another  set  of  accessory 
organs,  the  lacrimal  apparatus. 

The  first  step  in  the  development  of  the  eye  is  the  growth 
of  a  diverticulum  from  the  side  of  the  primary  fore-brain 
vesicle  (Fig.  1 44).     These  optic  evaginations  are  quite  large 


Fig.  144. — A,  brain  of  two-day  chick-embryo ;  B,  brain  of  human  embryo  of 
three  weeks  (His).  Shows  the  development  of  the  optic  vesicles  and  brain-vesi- 
cles ;  /&,  fore-brain ;  ih,  inter-brain ;  ov,  optic  vesicles. 


as  compared  with  the  brain-vesicle.  They  begin  to  be  evi- 
dent even  before  the  neural  tube  is  completely  closed.  As 
the  attached  part  of  the  diverticulum  expands  less  rapidly 
than  the  distal  portion,  the  evagination  soon  assumes  the 
form  of  a  sac  or  vesicle,  the  optic  vesicle,  connected  by  a  hol- 
low stalk  with  the  primary  fore-brain.  When  the  secondary 
fore-brain  vesicles  grow  out  anteriorly  from  the  primary  ves- 
icle, the  region  of  the  latter  that  becomes  in  consequence  the 
inter-brain  is  the  part  to  which  the  stalk  of  the  optic  vesicle 
is  attached.  Hence  the  optic  vesicle  is  an  appendage  of  the 
inter-brain  or  thalamencephalon  and  its  point  of  attachment 
to  the  latter  is  at  the  lateral  part  of  the  base,  in  front  of  the 
region  of  the  infundibulum  (Fig.  131,  A  and  C). 

The  optic  vesicle  expands  laterally  and  dorsally  until  it 
lies  immediately  beneath  the  epidermis,  forming  a  promi- 


304  TEXT-BOOK  OF  EMBRYOLOGY. 

nence  on  the  side  of  the  head  (Fig,  51).  The  ectoderm 
at  the  point  of  contact  with  the  optic  vesicle  becomes  thick- 
ened and  depressed,  the  differentiation  of  this  lens-area  being 
the  starting  point  of  the  crystalline  lens.  The  depressed 
patch  of  ectoderm,  sinking  more  deeply,  is  converted  into  a 
sac,  the  lens-vesicle,  the  connection  of  which  with  the  surface- 
cells  is  soon  lost.  The  distal  wall  of  the  optic  vesicle,  upon 
coming  into  contact  with  the  lens-vesicle,  undergoes  invagi- 
nation, this  wall  sinking  in  until  the  cavity  of  the  vesicle  is 
almost  obliterated.  Thus  the  vesicle  is  converted  into  the 
double-walled  optic  cup,  the  opening  of  which  looks  laterally 
toward  the  surface  of  the  head,  and  is  occupied  by  the  lens- 
vesicle. 

The  invaginated  wall  of  the  vesicle — that  is,  the  layer 
nearer  the  center  of  the  cup — becomes  the  retina,  except  its 
pigment-layer,  the  latter  resulting  from  the  outer  layer  of 
the  cup.  The  stalk  of  the  cup  becomes  the  optic  nerve. 
The  surrounding  mesodermic  tissue  grows  into  the  openings 
referred  to  above,  and  gives  rise  to  the  vitreous  humor, 
while  the  mesodermic  cells  that  closely  envelop  the  optic  cup 
produce  the  uveal  tract  and  the  sclera  and  cornea. 

Having  traced  briefly  the  development  of  the  organ,  its 
several  parts  may  now  be  considered  in  detail. 

The  Retina  and  the  Optic  Nerve. — These  two  struct- 
ures, as  stated  above,  are  directly  derived  from  the  optic 
vesicle  and  its  stalk. 

To  repeat,  for  the  sake  of  continuity,  some  points  already 
mentioned,  the  optic  vesicle  grows  forth  as  a  diverticulum 
from  the  side  of  the  primary  fore-brain  vesicle,  its  appear- 
ance being  foreshadowed  by  a  lateral  bulging  of  this  vesicle 
even  before  the  neural  canal  is  completely  closed.  When 
the  primary  fore-brain  vesicle  divides  into  the  secondary 
fore-brain  vesicles  and  the  vesicle  of  the  inter-brain,  the 
region  of  origin  of  the  optic  vesicle  falls  to  the  latter,  the 
point  of  attachment  being  at  the  outer  edge  of  the  base  of 
the  vesicle  in  front  of  the  infundibular  evagination.  The 
optic  nerve  is  to  be  regarded  therefore  as  springing  from  the 
inter-brain  or  thalamencephalon. 


THE  DEVELOPMENT  OF  THE  EYE, 


305 


The  outer  extremity  of  the  diverticulum  expanding  more 
rapidly  than  its  base  of  attachment  assumes  the  form  of  a 
vesicle  with  a  narrow  stalk  (Fig.  145),  the  stalked  condition 


Fig.  145.— Part  of  a  section  through  the  head  of  an  early  human  embryo,  show- 
ing the  connection  of  the  primary  optic  vesicles  with  the  fore-brain  (His) :  olf, 
olfactory  area  of  epiblast;  c.h.,  part  of  fore-brain  which  gives  rise  to  cerebral  hem- 
ispheres ;  th,  thalamencephalon ;  p.o.v.,  primary  optic  vesicles. 

being  present  in  the  fourth  week.  The  vesicles  grow  in  the 
outward  direction  and  form  a  prominence  on  each  side  of  the 
head.    There  being  no  brain-case  at  this  time,  they  lie  imme- 


Fig.  146. — Three  successive  stages  of  development  of  the  eye,  showing  forma- 
tion of  secondary  optic  cup  and  crystalline  lens  in  human  embryos  of  4  mm.  (.4), 
6  mm.  {B),  and  8  mm,  (C),  (Tourneux) :  a,  a,  primitive  optic  vesicles;  &,  external 
layer  of  secondary  optic  cup  (future  pigment-layer  of  retina) ;  c,  inner  layer  of  cup 
(retina  proper) ;  d,  lens-pit  (thickened  and  depressed  ectoderm) ;  e,  lens-vesicle. 

diately  under  the  epidermis,  separated  from  it  by  only  a  thin 
layer  of  embryonal  connective  tissue.  This  lateral  position 
of  the  optic  vesicles  is  characteristic  of  the  early  stages  of 
development.     After  the  end  of  the   first  month  the  eyes 

20 


306 


TEXT-BOOK  OF  EMBRYOLOGY. 


gradually  move  forward  and  downward  toward  their  perma- 
nent position,  which  is  approximately  attained  probably 
early  in  the  third  month. 

Shortly  after  the  fourth  week  the  distal  or  lateral  wall  and 
the  under  surface  of  the  optic  vesicle  become  invaginated. 
The  invagination  begins  when  the  vesicle  comes  into  contact 
with  the  lens-vesicle  (Fig.  146).  When  the  infolding  is 
complete,  the  vesicle  has  become  the  secondary  optic  cup, 
which  latter  consists  therefore  of  two  layers,  an  inner  and 
an  outer.  The  mouth  of  the  cup,  which  faces  away  from 
the  median  plane  of  the  head,  is  occupied  by  the  lens-vesicle. 
Since  the  under  surface  of  the  vesicle  participates  in  the  in- 
vaginating  process  (Fig.  147)  there  is  also  in  this  wall  of  the 
cup  an  aperture,  which  is  known  as  the  choroidal  fissure. 
The  invagination  likewise  aifects  the  under  surface  of  the 
tubular  stalk  of  the  vesicle  so  that  it  is  converted  into  an  in- 
verted double-layered  trough.     These  invaginations  bear  an 

important  relation  not  only  to  the 
further  metamorphosis  of  the  optic 
vesicle  and  its  stalk  into  the  retina 
and  the  optic  nerve,  but  also  to  the 
development  of  the  vitreous  body 
and  of  the  central  artery  of  the  re- 
tina. Thus,  the  vitreous  body  is 
produced  by  the  mesodermic  tis- 
sue that  finds  access  to  the  cup 
through  the  choroidal  fissure,  and 
the  arteria  centralis  retinae  is  de- 
veloped in  the  vascular  mesoder- 
mic tissue  that  invaginates  the 
under  surface  of  the  stalk  of  the 
vesicle. 

The  choroidal  fissure  gradually 
contracts  after  the  entrance  of  the 
mesoderm,  and  in  the  last  month  of  fetal  life  it  entirely  closes. 
The  mouth  of  the  optic  cup  embraces  the  lens,  its  rim  being 
always  on  the  distal  side  of,  or  superficial  to,  that  structure. 
This  opening  represents  the  pupil  of  later  stages. 


Fig.  147. — Plastic  representa- 
tion of  the  optic  cup  with  lens 
and  vitreous  body  (Hertwig) :  ab, 
outer  wall  of  the  cup ;  ib,  its  inner 
wall ;  h,  cavity  between  the  two 
walls,  which  later  disappears  en- 
tirely ;  Sn,  fundament  of  the  optic 
nerve  (stalk  of  the  optic  vesicle 
with  a  furrow  on  its  lower  sur- 
face) ;  aus,  optic  (choroid)  fis- 
sure ;  gl,  vitreous  body ;  I,  lens. 


THE  DEVELOPMENT  OF  THE  EYE.  307 

The  further  metamorphosis  of  the  optic  cup  includes  alter- 
ations peculiar  to  each  of  the  two  layers  and  also  to  the 
different  regions  of  the  cup.  The  mouth  of  the  cup  contracts 
somewhat  by  increased  growth  of  the  wall,  and  thus  there  is 
a  zone  bordering  this  orifice  which  is  anterior  to  the  lens, 
holding  the  same  relation  to  the  latter  body  that  the  future 
iris  holds.  A  second  zone  corresponds  with  the  periphery 
of  the  lens,  while  a  third  region,  the  fundus  of  the  cup, 
includes  all  the  remaining  part  of  its  wall. 

The  fundus  of  the  cup  undergoes  much  greater  specialization 
than  the  other  regions.  The  outer  layer  of  the  cup  remains 
thin,  consisting  of  a  single  layer  of  cells  which  assume  the 
cuboidal  form  and  become  infiltrated  with  pigment-granules. 
This  forms  the  pigment-layer  of  the  retina.  The  inner 
lamina  of  the  cup  thickens,  by  the  multiplication  of  its  cells, 
and  soon  consists  of  numerous  spindle-shaped  cells.  The 
thickened  fundus  is  marked  off  from  the  zone  that  surrounds 
the  periphery  of  the  lens  by  a  slight  groove  which  corres- 
ponds in  position  with  the  future  ora  serrata.  These  early 
spindle-cells  give  rise  to  two  kinds  of  elements,  the  stroma 
of  the  retina,  or  Miiller's  fibers,  and  the  various  nerve-cells, 
including  the  highly  specialized  rod-  and  cone-visual  cells. 

The  principal  sustentacular  elements,  or  Miiller's  fibers, 
like  the  spongioblasts  of  the  neural  tube,  are  radially 
arranged  and  extend  throughout  the  entire  thickness  of  the 
retina.  Their  inner  expanded  extremities,  in  close  contact 
with  each  other,  form  the  inner  limiting  membrane,  while 
their  outer  ends,  in  the  same  way,  constitute  the  outer  limit- 
ing membrane,  which  latter  is  in  contact  with  the  pigment- 
layer.  The  stroma  of  the  retina  receives  a  small  contribu- 
tion from  the  mesodermic  tissue,  which  ffrows  into  it  through 
the  choroidal  fissure  to  furnish  the  vascular  supply. 

Of  the  nerve-cells,  those  near  the  pigment-layer  undergo 
great  alteration  in  form  and  become  the  sensory  epithelium 
— that  is,  the  rod-  and  cone-visual  cells.  At  first  these  lie 
entirely  internal  to  the  external  limiting  membrane,  which 
separates  them  from  the  pigment-layer.  After  a  time,  how- 
ever, processes  grow  out — that  is,  away  from  the  center  of 


J08 


TEXT-BOOK  OF  EMBRYOLOGY. 


the  eyeball — and  perforate  the  external  limiting  membrane 
to  penetrate  between  the  cells  of  the  pigment-layer.  These 
processes  are  the  rods  and  cones,  and  collectively  constitute 
the  layer  of  rods  and  cones  of  the  adult.     The  bodies  of  the 


Fig.  148.— Section  through  the  optic  fundament  of  an  embryo  mouse  (after 
Kessler) :  pi,  pigmented  epithelium  of  the  eye  (outer  lamella  of  the  optic  cup,  or 
secondary  optic  vesicle) ;  r,  retina  (inner  lamella  of  the  optic  cup) ;  rz,  marginal  zone 
of  the  optic  cup,  which  forms  the  pars  ciliaris  et  iridis  retinse ;  g,  vitreous  body 
with  blood-vessels ;  tv,  tunica  vasculosa  lentls ;  hk,  blood-corpuscles ;  ch,  choroidea ; 
y,  lens-fibers ;  le,  lens-epithelium  ;  I',  zone  of  the  lens-fiber  nuclei ;  h,  fundament 
of  the  cornea;  he,  external  corneal  epithelium. 


rod-  and  cone-visual  cells,  situated  on  the  inner  side  of  the 
membrana  limitans  externa,  are  elongated  into  narrow  ele- 
ments, the  position  of  the  nuclei  being  indicated  by  slight 
enlargements.  They  constitute  the  outer  nuclear  layer  of 
the  mature  retina.     The  outer  nuclear  layer  and  the  layer  of 


THE  DEVELOPMENT  OF  THE  EYE.  309 

rods  and  cones  are  to  be  regarded,  therefore,  as  one  layer  of 
highly  specialized  neuro-epithelium,  made  up  of  the  rod- 
visual  cells  and  the  cone-visual  cells,  the  inner  segments  or 
bodies  of  the  cells  being  only  apparently  isolated  from  the 
outer  segments,  the  rods  and  cones  respectively,  by  the  fact 
that  the  latter  project  through  minute  apertures  in  the 
external  limiting  membrane.  The  axis-cylinder  processes  of 
these  cells  pass  toward  the  center  of  the  eyeball. 

The  neuro-epithelium  of  the  retina  is  the  last  of  its  elements 
to  develop.  In  man  and  in  many  mammals,  it  is  present  at 
birth.  In  the  cat  and  the  rabbit,  the  rod-  and  cone-visual 
cells  develop  after  birth,  and  hence  the  new-born  of  these 
species  are  blind.    The  macula  lutea  is  developed  after  birth. 

The  cells  of  the  inner  part  of  the  retina  differentiate  into 
the  remaining  nervous  elements,  some  becoming  the  bipolar 
and  other  cells  of  the  inner  nuclear  layer — the  ganglion 
retinse — while  others  form  the  large  ganglion  cells  of  the 
ganglion-cell  layer.  The  axis-cylinder  processes  of  the 
ganglion  cells  are  directed  inward  to  form  the  nerve-fiber 
layer,  the  fibers  of  this  layer  converging  from  all  parts  of 
the  inner  surface  of  the  retina  toward  the  optic  disk  or 
papilla.  Here  they  perforate  the  retina,  as  well  as  the  cho- 
roid and  sclera,  to  pass,  as  optic  nerve-fibers,  to  the  brain. 

This  part  of  the  optic  cup,  the  fundus,  produces  then,  in 
the  manner  described  above,  the  functionating  portion  of  the 
retina,  or  the  pars  optica  retinae,  the  anterior  termination  of 
which  is  indicated  by  the  orra  serrata. 

The  lenticular  zone  of  the  optic  cup,  which  is  in  relation 
with  the  periphery  of  the  lens,  undergoes  comparatively 
slight  specialization.  Its  outer  lamella  is  pigmented,  as  in 
the  fundus  of  the  cup.  Its  inner  layer  remains  very  thin 
and  consists  of  cells  which  at  first  are  cuboidal,  but  which 
later  become  cylindrical.  At  the  end  of  the  second  month, 
or  the  beginning  of  the  third,  the  two  layers  of  the  lenticular 
zone  become  plicated,  owing  to  excessive  growth  in  super- 
ficial extent.  The  folds  are  nearly  parallel  and  are  arranged 
radially  with  reference  to  the  lens,  the  margin  of  which  they 
surround.     These  folds  are  the  first  indication  of  the  ciliary 


310  TEXT-BOOK  OF  EMBRYOLOGY. 

processes.  The  mesodermic  tissue  immediately  external  to 
the  optic  cup  diflPerentiates  into  the  uveal  tract^  the  part  cor- 
responding with  the  lenticular  zone  of  the  cup  furnishing  the 
ciliary  body.  The  young  growing  connective  tissue  pene- 
trates between  the  folds  of  the  lenticular  zone  of  the  cup, 
acquiring  intimate  union  with  the  pigment-layer,  and  thus 
provides  the  connective -tissue  basis  of  the  ciliary  processes. 
This  lenticular  zone  of  the  two  layers  of  the  optic  cup, 
therefore,  constitutes  the  lining,  or  internal  covering,  of  the 
ciliary  body,  and  must  necessarily  be  regarded  as  the  contin- 
uation of  the  retina.  It  is  known  as  the  pars  ciliaris  retinae 
of  the  fully  developed  eye. 

The  marginal  zone  of  the  optic  cup,  or  the  region  border- 
ing its  orifice,  is  also  related  in  its  further  growth  with  the 
uveal  tract.  Although  in  the  earlier  stages  of  development 
the  lens  lies  in  the  mouth  of  the  cup,  as  time  goes  on  the 
relation  is  so  altered  that  the  aperture  and  the  zone  which 
borders  it  occupy  a  position  in  front  of  the  lens.  In  this 
marginal  zone  both  lamellae  of  the  cup  become  pigmented 
and  acquire  union  with  the  layer  of  mesodermic  tissue  which 
is  differentiating  into  the  iris,  and  they  therefore  contribute 
to  the  formation  of  that  structure,  constituting  its  pigment- 
layer.  The  pigment-layer  of  the  posterier  surface  of  the 
iris  is,  therefore,  an  extended  but  rudimentary  part  of  the 
retina.     It  is  called  the  pars  iridica  retinae. 

From  what  has  been  said,  it  will  be  apparent  that  the 
retina  forms  a  complete  tunic  with  an  anterior  perforation, 
the  pupil,  and  that  it  consists  of  the  functionally  active  part, 
or  retina  proper,  the  pars  optica  retinae ;  of  the  pars  ciliaris 
retinae,  marked  ofP  from  the  latter  by  the  ora  serrata ;  and 
of  the  pars  iridica  retinae,  which  terminates  at  the  margin 
of  the  pupil. 

The  evolution  of  the  optic  cup  or  secondary  optic  vesicle 
may  be  thus  summarized : 

I.  Marginal  or  most  anterior        The  thin   atrophic   pars  iridica  re- 
region  of  cup.  tinse,  or  pigment-layer  of  the  iris. 
II.  Lenticular  zone  of  cup.  Pars  ciliaris  retinae,  covering  inner 

surface  of  ciliary  body. 


THE  DEVELOPMENT  OF  THE  EYE.  311 

III.  Fundus  of  cup.  Functionating  part  of  retina,  or  pars 

optica  retinae,  including  : 

A.  Outer  layer.  A.  Pigment-layer  of  retina. 

B.  Inner  layer.  B.  1.  Neuro-epithelial   layer,    made 

up  of  layer  of  rods  and  cones  (the  pro- 
cesses of  the  rod-  and  cone-visual  cells) ; 
membrana  limitans  externa ;  outer 
nuclear  layer  (the  bodies  of  the  rod- 
and  cone-cells). 

2.  Cerebral  layer  (representing  an 
interpolated  ganglion  with  connecting 
fibers),  consisting  of : 

Outer  reticular  layer ; 
*  Inner  nuclear  layer ; 

Inner  reticular  layer; 

Ganglion-cell  layer; 

Nerve-fiber  laver. 


The  optic  nerve  is  the  metamorphosed  stalk  of  the  optic  ves- 
icle. When  the  distal  and  under  surfaces  of  the  vesicle  suffer 
invagination,  the  stalk  participates  in  the  process,  its  under 
surface  being  marked  by  a  groove  which  is  a  prolongation  of 
the  choroidal  fissure  of  the  optic  cup  (Fig.  147).  By  this  in- 
folding, the  cavity  of  the  stalk  is  obliterated  and  the  stalk  is 
converted  into  a  double-walled  tube  enclosing  mesodermic 
tissue  which  follows  the  invaginating  ventral  wall.  In  this 
mesodermic  tissue  is  developed  the  arteria  centralis  retinae. 
In  mammals  the  invagination  affects  only  the  distal  part  of 
the  stalk,  the  segment  included  between  the  eyeball  and  the 
point  corresponding  in  the  adult  to  the  place  of  entrance  into 
the  nerve  of  the  central  artery.  It  must  be  apparent  that 
the  outer  layer  of  the  tube  thus  formed  is  directly  continuous 
with  the  outer  layer  of  the  optic  cup,  while  the  invaginated 
lamina  is  the  prolongation  of  the  inner  wall  of  the  cup  or 
of  the  part  that  becomes  the  retina  proper,  since  not  only 
the  distal  wall  of  the  optic  vesicle  is  invaginated,  but  its 
under  or  ventral  wall  as  well. 

The  primitive  optic  nerve  at  this  stage  consists  of  layers 
of  spindle-shaped  cells,  with  a  central  core  of  vascular  con- 
nective tissue. 

The  manner  in  which  the  nerve-fibers  are  developed  is 


312  TEXT-BOOK  OF  EMBRYOLOGY. 

still  a  matter  of  controversy.  According  to  His  and  Kdlli- 
ker,  the  fibers  grow  out  from  the  ganglion-cells  of  the  optic 
thalami  and  the  anterior  corpora  quadrigemina,  while  Miiller 
believes  that  they  are  the  prolonged  axis-cylinder  processes 
of  the  ganglion-cells  of  the  retina.  In  either  case  the  cells 
of  the  optic  stalk  Avould  furnish  only  the  sustentative  tissue 
of  the  nerve.  There  is  also  a  contribution  of  sustentative 
tissue  or  stroma  from  the  mesoderm,  as  in  the  case  of  the 
central  nervous  system. 

The  Crystalline  I/ens. — The  lens,  exclusive  of  its  cap- 
sule, is,  like  the  retina,  of  ectodermic  origin.  The  first  step 
in  its  development  is  the  formation  of  a  thickened  and  de- 
pressed patch  of  the  ectoderm  on  the  lateral  surface  of  the 
head,  this  area  being  situated  at  the  place  where  the  optic 
vesicle  is  nearest  the  surface  (Fig.  146,  B,  d).  The  de- 
pression is  the  lens-pit.  It  soon  becomes  converted  into  a 
closed  sac,  the  lens-vesicle,  by  the  gradual  approximation  and 
union  of  its  edges.  The  pit  receding  from  the  surface  as  its 
lips  come  together,  the  completed  vesicle  lies  under  the  sur- 
face ectoderm,  with  which  it  is  for  a  time  connected  by  the 
slender  stalk  of  the  invagination.  Upon  the  disappear- 
ance of  the  strand  of  cells  constituting  the  stalk,  the  lens- 
vesicle  is  completely  isolated  from  the  outer  germ-layer 
(Fig.  146,  ^.  ^)-     _       _        ^ 

■  The  lens-vesicle  in  birds  is  a  hollow  epithelial  sac  several 
layers  thick,  but  in  mammals  the  central  cavity  contains  a 
mass  of  cells,  which  latter  disappear  in  the  later  stages  of 
development. 

Upon  the  invagination  of  the  optic  vesicle  to  form  the 
secondary  optic  cup,  the  lens-vesicle  is  embraced  by  the  lips 
of  the  cu])  and  still  later  comes  to  lie  within  the  cup,  near  its 
orifice  (Fig.  148). 

The  further  alterations  in  the  vesicle  are  dependent  pri- 
marily upon  changes  in  its  deep  and  superficial  walls  re- 
spectively, each  of  which  consists  of  several  layers  of  cylin- 
drical cells.  The  cells  of  the  superficial  wall  alter  their 
form,  becoming  cuboidal,  while  the  posterior  or  deeper  cells 
lengthen  so  as  to  become  fibers.     Thus  the  deeper  wall  of 


THE  CRYSTALLINE  LENS.  313 

the  vesicle  thickens  at  the  expense  of  the  central  cavity — the 
central  mass  of  cells  at  the  same  time  disappearing — while 
the  superficial  layer  remains  thin.  The  two  strata  are  con- 
tinuous with  each  other  at  the  equator  of  the  lens,  one  form 
gradually  merging  into  the  other  at  this  region,  which  is  a 
zone  of  transition  (Fig.  148). 

The  lens  at  this  stage  is  composed,  therefore,  of  a  thin 
superficial  or  anterior  stratum  of  cuboidal  epithelial  cells  and 
a  much  thicker  posterior  or  deep  layer  of  so-called  fibers,  the 
latter  being  simply  the  greatly  elongated  cells  of  the  posterior 
wall  of  the  vesicle.  Between  the  two  laminae  is  a  small 
remnant  of  the  cavity  of  the  vesicle.  The  epithelial  layer 
persists  throughout  life  as  the  epithelium  of  the  lens,  while 
the  fibrous  layer  is  the  basis  of  the  lens-fibers  of  the  mature 
condition.  The  cavity  sometimes  persists  as  a  small  space 
containing  a  few  drops  of  fluid,  the  liquor  of  Morgagni. 

The  next  important  stage  in  the  development  of  the  lens 
is  the  formation  of  additional  lens-fibers.  These  result  from 
the  proliferation  of  the  cells  of  the  epithelial  or  anterior 
layer.  The  lens-fibers  are  formed  in  successive  layers,  as 
may  be  made  evident  by  the  maceration  of  a  lens.  Each 
fiber  extends  from  the  anterior  to  the  posterior  surface  of  the 
lens.  The  ends  of  the  fibers  meet  each  other  alonar  regular 
lines,  producing  thus  the  characteristic  three-rayed  figures  or 
stars  of  the  lens,  one  of  which  belongs  to  each  surface. 
Hence,  while  the  lens-fibers  first  formed  are  the  elongated 
cells  of  the  posterior  layer  of  the  lens-vesicle,  the  fibers  of 
later  growth  originate  from  the  cells  of  the  anterior  wall. 
The  epithelial  character  of  the  lens-fibers  is  evinced  by  the 
presence  of  a  nucleus  in  each  fiber  of  a  young  lens. 

The  lens-capsule  results  from  the  diiferentiation  of  the 
mesodermic  tissue  which  surrounds  the  lens.  It  is  from  this 
enveloping  vascular  lamina,  the  tunica  vasculosa  lentis,  that 
the  growing  lens  derives  its  nutrition.  The  capsule  is  well 
marked  in  the  second  month.  Its  blood-vessels  are  derived 
from  those  of  the  vitreous  body.  At  the  end  of  the  seventh 
month  this  well-developed,  highly  vascular  membrane  begins 
to  undergo  retrograde  alterations,  the  final  result  of  which  is 


314  TEXT-BOOK  OF  EMBRYOLOGY. 

its  transformation  into  the  thin,  non-vascular,  transparent 
capsule  of  the  mature  lens.^  The  most  active  growth  of  the  lens 
itself  occurs  prior  to  the  degeneration  of  the  tunica  vasculosa 
lentis,  so  that  even  before  the  end  of  fetal  life  the  lens  has 
nearly  attained  its  full  size.  Thus  the  weight  of  the  lens  of 
the  new-born  child  is  123  milligrammes,  while  that  of  the 
adult  lens  is  but  190  milligrammes  (Huschke). 

Hence  the  crystalline  lens  has  a  double  origin,  the  lens-sub- 
stance or  lens  proper  being  derived  from  the  ectoderm,  while 
the  capsule  originates  from  the  mesoderm. 

The  Vitreous  Body. — The  vitreous  body,  representing  a 
comparatively  slightly  differentiated  form  of  connective  tissue, 
is  derived  from  the  middle  germ-layer.  The  mesodermic 
tissue,  already  in  the  stage  of  embryonal  connective  tissue, 
gains  access  to  the  optic  cup  through  the  choroidal  fissure 
(Fig.  147),  its  ingrowth  in  fact  accompanying  the  invagina- 
tion of  the  under  surface  of  the  optic  vesicle.  Since  the 
inferior  surface  of  the  stalk  of  the  vesicle — the  future  optic 
nerve — participates  in  the  invagination,  the  mass  of  meso- 
dermic tissue  which  gives  rise  to  the  vitreous  is  continuous 
with  that  which  invaginates  the  primitive  optic  nerve  to 
produce  the  central  artery  of  the  retina.  As  a  consequence, 
the  blood-vessels  which  soon  develop  so  plentifully  in  the 
vitreous  body  are  extensions  from  the  central  artery  of  the 
retina,  the  latter  itself  being  continued  forward  as  the  hyaloid 
artery.  The  terminal  branches  of  the  hyaloid  artery  pass  on 
through  the  vitreous  body  to  terminate  in  the  vascular  cap- 
sule of  the  growing  lens,  constituting  the  blood-supply  of 
that  structure. 

The  intercellular  substance  of  the  young  tissue  undergoes 
but  little  differentiation,  while  the  cells  become  gradually 
reduced  to  a  few  stellate  elements  which  ultimately  entirely 
disappear.     The  peripheral  part  of  the  tissue  develops  into 

^  It  sometimes  happens  that  parts  of  tlie  fetal  lens-capsule  persist.  The 
most  common  example  of  such  persistence  is  the  so-called  membrana  pupil- 
laris  sometimes  present  at  birth,  producing  confjenital  atresia  of  the  pupil. 
This  results  from  the  persistence  of  that  part  of  the  fetal  capsule  which  is 
situated  on  the  anterior  surface  of  the  lens,  behind  the  pupil. 


THE  MIDDLE  AND   OUTER   TUNICS  OF  THE  EYE.   315 

the  hyaloid  membrane,  which  anteriorly  acquires  union  with 
the  capsule  of  the  lens. 

The  blood-vessels  of  the  vitreous  disappear  during  the  last 
two  or  three  months  of  fetal  life.  The  hyaloid  artery  per- 
sists, although  in  reduced  form,  for  a  longer  time  than  the 
smaller  vessels.  Upon  its  final  degeneration  it  is  replaced 
by  a  canal,  the  hyaloid  canal,  or  canal  of  Stilling,  which  is 
present  in  adult  life. 

The  Middle  or  Vascular  and  the  Outer  or  Fibrous 
Tunics  of  the  l^yQ. — The  outer  fibrous  coat  of  the  eye,  in- 
cluding the  sclera  and  the  cornea,  and  the  middle  tunic  or 
uveal  tract,  comprising  the  choroid,  the  ciliary  body,  and  the 
iris,  are  structures  of  mesodermic  origin,  being  directly  pro- 
duced by  the  mesodermic  tissue  surrounding  the  optic  cup. 
The  richly  cellular  mesoderm  applies  itself  to  the  exterior  of 
the  cup  and  diiferentiates  into  the  two  layers  in  question,  the 
changes  involving  on  the  one  hand  the  metamorphosis  of  the 
mesodermic  cells  chiefly  into  muscular  and  vascular  elements, 
and  on  the  other  hand  the  evolution  of  a  tissue  essentially 
fibrous  in  structure.  These  two  tunics  are  distinguishable 
in  the  sixth  week. 

The  cornea  is  formed  from  the  thin  layer  of  mesoderm  that 
penetrates  between  the  lens-vesicle  and  the  surface  ectoderm. 
The  lens-vesicle  lies  very  near  the  surface,  and  the  thin 
stratum  of  mesoderm  that  is  interposed  between  the  two  is 
the  anterior  layer  of  the  lens-capsule  (Fig.  148).  This  ante- 
rior layer  thickens  by  the  immigration  of  other  cells  and  sub- 
sequently splits  into  two  laminae,  a  superficial  one  which  pro- 
duces the  cornea  (Fig.  148,  h),  and  a  deeper,  which  is  now  the 
proper  anterior  wall  of  the  lens-capsule.  Thus  a  space  filled 
with  fluid  appears  between  the  primitive  cornea  and  the  lens, 
which  corresponds  with  the  future  anterior  and  posterior 
chambers  of  the  eye,  the  division  of  the  space  into  these  two 
chambers  being  effected  subsequently  by  the  development  of 
the  iris.  The  further  development  of  the  cornea  consists 
simply  in  the  differentiation  of  the  mesodermic  cells  and  the 
intercellular  substance  into  the  several  characteristic  elements 
of  the  adult  structure. 


316  TEXT-BOOK  OF  EMBRYOLOGY. 

The  uveal  tract  closely  corresponds  in  extent  with  the  two 
layers  of  the  optic  cup.  The  choroid  is  differentiated  from 
that  portion  of  this  primitive  uveal  tract  which  envelops  the 
pars  optica  of  the  retina.  In  this  region  the  enveloping 
layer  of  mesodermic  cells  develops  into  the  several  elements 
of  the  choroid,  the  most  conspicuous  of  which  are  an  inner 
layer  of  capillary  vessels,  the  choriocapillaris,  and  an  outer 
layer  of  larger  vessels,  the  stroma-layer  of  the  choroid.  The 
development  of  the  choroid  bears  a  certain  relation  to  the 
choroidal  fissure  of  the  optic  cup.  This  fissure  has  been  re- 
ferred to  as  a  gap  in  the  under  surface  of  the  cup  corre- 
sponding with  the  line  of  invagination  through  which  the 
mesodermic  tissue,  of  which  the  developing  choroid  is  a 
part,  grows  into  the  cup  to  produce  the  vitreous.  Although 
normally  this  fissure  in  the  retina  entirely  disappears,  its  site 
becomes  pigmented  later  than  other  regions  of  the  pigment- 
layer  of  the  retina,  and  hence  there  is,  for  a  time,  a  clear 
streak  in  this  part  of  the  retina  which  has  the  appearance  of 
a  fissure  in  that  membrane.  As  the  pigment-layer  of  the 
retina  was  formerly  assigned  to  the  choroid,  this  streak  ap- 
peared to  be  a  breach  of  continuity  of  the  choroid  ;  hence  the 
term  choroidal  fissure.  In  some  cases,  however,  the  choroidal 
fissure  fails  to  close,  and  as  the  development  of  the  choroid 
is  largely  dependent  upon  or  is  governed  by  that  of  the 
retina  there  remains  a  corresponding  gap  in  the  choroid. 
This  defect  enables  the  sclera  to  be  seen  from  the  interior  in 
a  line  extending  forward  from  the  optic  nerve  entrance.  It 
is  known  as  coloboma  of  the  choroid. 

The  ciliary  body  is  developed  immediately  in  advance  of 
the  choroid  and  from  the  same  layer  of  mesodermic  tissue. 
The  deeper  parts  of  the  tissue  in  this  region  correspond  with 
the  plications  of  the  ciliary  part  of  the  retina,  sending  proc- 
esses into  and  between  the  radial  folds  of  this  part  of  the 
two  layers  of  the  optic  cup,  with  which  latter  the  highly 
vascular  mesodermic  tissue  acquires  firm  union.  This  results 
in  the  formation  of  the  ciliary  processes.  Some  of  the  cells 
of  the  more  peripheral  part  of  this  zone  are  converted  into 
unstriated  muscular  tissue,  thus  producing  the  ciliary  muscle. 


THE  MIDDLE  AND   OUTER   TUNICS  OF  THE  EYE.   317 

All  the  characteristic  or  important  elements  of  the  ciliary 
body  are,  therefore,  derived  from  the  mesoderm,  while  the 
thin  layer  of  tissue  on  its  inner  surface,  representing  an 
undeveloped  part  of  the  optic  cup,  the  pars  ciliaris,  is  of  ecto- 
dermic  origin. 

The  iris,  the  most  anterior  zone  of  the  uveal  tunic,  is  pro- 
duced from  the  same  mesodermic  tract  that  gives  rise  to  the 


.  Pig.  149— Sagittal  section  through  the  eye  of  an  embryo  rabbit  of  eighteen 
days  X  30  (KoUiker) :  o,  optic  nerve ;  p,  hexagonal  pigment- layer  ;  r,  retina ;  re, 
ciliary  part  of  the  retina  ;  p',  forepart  of  the  optic  cup  (rudiment  of  the  iris-pig- 
ment) ;  g,  vitreous,  shrunk  away  from  the  retina,  except  where  the  vessels  from 
the  arteria  centralis  retinae  enter  it ;  i,  iris ;  mp,  membrana  pupillaris ;  c,  cornea 
with  epithelium  e;  pp,  pa,  palpebrae ;  i,  lens  ;  I',  lens-epithelium;  /,  sclerotic;  m, 
recti  muscles. 

choroid  and  to  the  ciliary  body.  As  stated  above,  soon  after 
the  lens-vesicle  becomes  constricted  off  from  the  surface  ecto- 
derm, it  is  enveloped  by  a  mass  of  mesodermic  cells  which 
constitute  its  primitive  capsule,  and  the  layer  of  these  cells 
lying  between  the  lens-vesicle  and  the  surface  ectoderm  splits 
into  an  anterior  layer,  which  becomes  the  cornea,  and  a  pos- 


318  TEXT-BOOK  OF  EMBRYOLOGY. 

terior  stratum  which  is  the  anterior  wall  of  the  lens-capsule. 
This  produces  a  space  between  the  lens  and  the  cornea.  The 
lens  now  recedes  farther  from  the  surface,  and  the  margins 
of  the  optic  cup  advance,  so  that  the  lens  now  lies  within 
the  cup,  the  marginal  zone  of  the  cup  being  in  front  of  the 
lens,  between  it  and  the  cornea,  while  its  equator  is  in  close 
relation  with  the  ciliary  regions  of  the  cup  and  of  the  uveal 
tract.  Thus  the  space  between  the  lens  and  the  cornea  is 
divided  into  an  anterior  compartment,  the  anterior  chamber, 
and  a  posterior  space,  the  posterior  chamber,  the  orifice  of 
the  cup  being  a  means  of  communication  between  the  two 
and  representing  the  pupil  of  a  later  stage.  The  marginal 
zone  of  the  cup  furnishes  the  guiding  line  for  the  develop- 
ment of  the  iris.  The  mesodermic  tissue  in  relation  with 
the  outer  surface  of  the  marginal  zone  of  the  cup  differen- 
tiates into  the  vascular,  muscular,  and  connective-tissue  ele- 
ments of  the  iris  proper,  while  its  posterior  pigment-layer  is 
constituted  by  the  slightly  specialized  layers  of  the  most 
anterior  part  of  the  optic  cup,  the  part  that  is  known  as  the, 
pars  iridica  retinse. 

Since  the  anterior  and  posterior  chambers  of  the  eye  are 
spaces  hollowed  out  of  the  mesoderm,  they  represent  a 
lymph-space  and  are,  as  such,  lined  with  endothelial  cells. 

The  cleft  in  the  inferior  wall  of  the  optic  cup  referred  to 
above  as  the  choroidal  fissure  necessarily  afPects  the  mar- 
ginal zone  of  the  cup  as  well  as  the  region  posterior  to  it. 
If  this  part  of  the  fissure  persists,  as  it  sometimes  does,  it 
may  be  accompanied  by  a  corresponding  deficiency  in  the 
tissues  of  the  iris  proper.  Such  a  congenital  defect,  appear- 
ing as  a  radial  cleft  in  the  lower  half  of  the  iris,  is  known  as 
coloboma  of  the  iris. 

The  Eyelids  and  the  I/acrimal  Apparatus. — The 
eyelids  are  developed  from  folds  of  the  primitive  epi- 
dermis that  form  over  the  superficial  part  of  the  developing 
eyeball  (Fig.  149,  pp  and  pa).  After  the  separation  of  the 
lens-vesicle  from  the  surface  ectoderm,  the  latter  pouches 
out  into  two  little  transverse  folds  for  the  upper  and  lower 
lids  respectively.     Each  fold  includes  a  certain  quantity  of 


THE  EYELIDS  AND  THE  LACRIMAL  APPARATUS.  319 

mesodermic  tissue,  from  which  are  produced  the  connective- 
tissue  elements  of  the  lids,  as  the  tarsal  plates,  etc.  After 
the  folds  attain  to  a  certain  degree  of  development  their 
edges  approach  each  other  and  become  adherent,  thus  enclos- 
ing a  space  between  the  primitive  lids  and  the  front  of  the 
eyeball.  The  infolded  ectodermic  layers  lining  this  space 
acquire  the  characteristic  features  of  mucous  membrane  and 
constitute  the  epithelium  of  the  conjunctiva,  the  part  of  this 
membrane  that  covers  the  cornea  adhering  closely  to  that 
structure  as  its  anterior  epithelial  layer.  The  union  of  the 
edges  of  the  lids  begins  in  the  third  month  and  lasts  until 
near  the  close  of  fetal  life.  A  short  time  before  birth  the 
permanent  palpebral  fissure  begins  to  form  by  the  breaking 
down  of  the  adhesions. 

A  part  of  the  mesodermic  tissue  of  the  lids  undergoes  con- 
version into  fibrous  connective  tissue,  thus  producing  the 
tarsal  plates  of  the  upper  and  lower  lids,  with  the  palpebral 
fasciae  and  tarsal  ligaments  by  which  the  plates  are  attached 
to  the  margins  of  the  orbit. 

During  the  period  when  the  edges  of  the  lids  are  adherent, 
the  Meibomian  glands  and  the  eye-lashes  are  formed.  Tlie 
glands  develop  from  solid  cords  of  epithelial  cells  that  grow 
from  the  deepest  or  Malpighian  layer  of  the  primitive  epi- 
dermis into  the  tarsal  plates.  The  cords  become  hollow 
tubes  by  degeneration  of  their  central  cells. 

In.  addition  to  the  two  principal  folds  that  produce  the 
lids,  a  third,  vertical  fold  appears  at  the  inner,  nasal  side  of 
the  conjunctival  space,  beneath  the  lids.  This  fold  remains 
quite  small  in  man  and  forms  the  plica  semilunaris,  but  in 
most  other  vertebrates  it  attains  much  greater  size  as  the 
third  eyelid  or  nictitating  membrane.  A  small  part  of  this 
third  fold  develops  sebaceous  glands  and  a  few  hair-follicles 
and  becomes  the  lacrimal  caruncle. 

The  lacrimal  gland  is  developed  in  the  same  manner  as 
tlie  Meibomian  glands,  by  the  growth  of  solid  epithelial 
cords  from  the  conjunctiva.  The  cords  grow  into  the  under- 
lying mesoderm  at  the  outer  part  of  the  line  of  reflection  of 
the  conjunctiva  from  the  inner  surface  of  the  upper  lid  to  the 


320  TEXT-BOOK  OF  EMBRYOLOGY. 

front  of  the  eyeball.  The  cords  acquire  lateral  branches  and 
then  become  hollowed  out  to  form  the  secreting  tubules  and 
efferent  ducts  of  the  gland,  the  connective-tissue  stroma  of 
which  is  contributed  by  the  surrounding  mesoderraic  tissue. 
The  orifices  of  the  adult  eflPerent  ducts  in  the  upper  outer 
part  of  the  conjunctival  sac  correspond  with  the  points  from 
Avhich  the  primitive  cell-cords  first  grow  forth. 

The  efferent  lacrimal  apparatus,  consisting  of  the  nasal 
or  lacrimal  duct  and  the  canaliculi,  is  related  genetically 
to  the  growth  of  the  nose  and  the  upper  jaw.  Soon  after 
the  appearance  of  the  nasofrontal  process,  a  lateral  projec- 
tion, the  lateral  nasal  process,  grows  from  its  side  near  the 
base  and  advances  downward  so  as  to  form  the  outer  bound- 
ary of  the  nasal  pit  and  consequently  of  the  future  nostril 
(Fig.  56,  A,  B).  This  lateral  nasal  process  is  separated  from 
the  maxillary  process  of  the  first  visceral  arch  by  an  oblique 
furroAV,  the  naso-optic  groove,  which  extends  from  the  inner 
angle  of  the  orbit  to  the  outer  side  of  the  nostril,  or,  before 
the  separation  of  the  nasal  pit  from  the  primitive  mouth,  to 
the  upper,  boundary  of  the  latter  orifice.  The  naso-optic 
groove  indicates  the  situation  of  the  lacrimal  duct.  By 
some  authorities — Coste  and  K5lliker — it  is  believed  that 
the  duct  results  from  the  union  of  the  edges  of  the  groove. 
Later  investigations  seem  to  indicate,  however,  that  the 
duct  is  formed  by  the  hollowing  out  of  a  solid  cord  of  epi- 
thelial cells  that  appears  at  the  bottom  of  the  furrow.  In 
either  case  the  epithelial  lining  of  the  duct  is  an  ectodermic 
involution.  When  the  nostrils  are  separated  from  the  oral 
aperture  by  the  union  of  the  nasofrontal,  the  lateral  nasal, 
and  the  maxillary  processes  (p.  121),  the  lower  end  of  the 
furrow  is  obliterated,  and  the  partially  formed  duct  is  made 
to  terminate  in  the  nasal  cavity. 

The  canaliculi,  representing  the  bifurcated  upper  extrem- 
ity of  the  duct,  result,  according  to  one  view,  from  the 
division  of  the  upper  end  of  the  epithelial  cord  into  two 
limbs,  one  for  each  lid,  and  their  subsequent  hollowing-out  ; 
according  to  another,  from  the  continuation  of  the  cell-cord 
into  the  upper  lid  and  the  later  addition  of  a  limb  for  the 


THE  DEVELOPMENT  OF  THE  ORGAN  OF  HEARING.    321 

canaliculus  of  the  lower  lid.     The  lacrimal  sac  is  merely  an 
expanded  part  of  the  duct. 

THE  DEVELOPMENT  OF  THE  ORGAN  OF  HEARING. 

As  in  the  case  of  the  other  sense-organs,  the  auditory- 
apparatus  consists  of  highly  specialized  neuro-epithelium, 
connected  by  nerve-fibers  and  interpolated  ganglia  with  the 
central  nervous  system,  and  of  protective  and  auxiliary- 
structures.  The  neuro-epithelial  structures,  including  the 
organ  of  Corti  and  the  cells  of  the  cristse  and  maculae 
acusticse,  result  from  the  specialization  of  certain  of  the 
epithelial  cells  which  line  the  membranous  labyrinth.  The 
perilymphatic  space,  which  is  a  lymph-space,  together  with 
its  bony  walls,  the  osseous  labyrinth,  serve  for  the  protec- 


FiG.  150.— Three  transverse  sections  showing  development  of  otic  vesicle  of 
human  embryo  (Tourneux) :  A,  from  embryo  of  3  mm.,  showing  auditory  pit;  B^ 
from  embryo  of  4  mm.,  showing  the  transformation  of  the  pit  into  the  otic  vesicle  , 
C,  from  embryo  of  6  mm.,  showing  otic  vesicle  detached  from  surface  ectoderm,, 
and  presenting  a  posterior  diverticulum,  the  recessus  vestibuli. 

tion  of  the  delicate  neural  elements,  while  the  middle  ear 
and  the  external  ear  act  as  media  for  the  conduction  of 
sonorous  vibrations. 

The  internal  ear  being  the  essential  part  of  the  organ  of 
hearing  and  being  also  the  part  first  formed  may  properly 
receive  first  consideration. 

The  Internal  i^ar. — The  membranous  labyrinth  of  the 

21 


322 


TEXT-BOOK  OF  EMBRYOLOGY. 


internal  ear  is  the  oldest  part  of  the  organ  of  hearing.  Its 
origin  is  from  a  thickened  circular  patch  of  ectoderm  on  the 
dorsolateral  surface  of  the  head-region  of  the  embryo  near 
the  dorsal  termination  of  the  first  outer  visceral  furrow.  The 
thickened  area  sinks  below  the  surface,  forming  thus  the 
auditory  pit,  which  is  present  in  the  third  week  (Fig.  150,  A). 
The  pit  becomes  deeper,  its  edges  approach  each  other  and 
finally  meet  and  unite  to  form  the  otic  vesicle  or  otocyst. 
This  little  epithelial  sac  gradually  recedes  from  the  surface 
ectoderm.  At  this  stage  of  development  there  is  no  cranial 
capsule  other  than  the  indifferent  mesodermic  tissue  which 
surrounds  the  brain- vesicles ;    hence,  the  otic  vesicle,  em- 


FiG.  151.— Development  of  the  membranous  labyrinth  of  the  human  ear  (W. 
His,  Jr.) :  A,  left  labyrinth  of  embryo  of  about  four  weeks,  outer  side ;  vc,  vesti- 
bular and  cochlear  portions ;  rl,  recessus  labyrinthi.  B,  left  labyrinth  with  parts 
of  facial  and  auditory  nerves  of  embryo  of  about  four  and  a  half  weeks  ;  rl,  reces- 
sus labyrinthi ;  ssc,  psc,  esc,  superior,  posterior,  and  external  semicircular  canals ; 
s,  saccule;  c,  cochlea;  1)71,/?!,  vestibular  and  facial  nerves;  vg,  eg,  gg,vestihula,r, 
cochlear,  and  geniculate  ganglia.  C,  left  labyrinth  of  embryo  of  about  five  weeks, 
from  without  and  below ;  labelling  as  in  preceding  figure. 

bedded  in  this  tissue,  lies  in  close  proximity  to  the  after- 
brain,  and  comes  into  relation  with  the  acusticofacial  gan- 
glion (p.  297).  The  vesicle,  at  first  spherical,  soon  becomes 
pear-shaped  owing  to  the  protrusion  of  its  dorsal  wall.  This 
dorsal  projection,  the  recessus  vestibuli  or  labyrinthi  (Fig. 


THE  INTERNAL  EAR.  323 

150,  (7),  lengthens  out  into  a  slender  tube,  the  ductus  endo- 
lympliaticus  (Fig.  152),  the  slightly  dilated  end  of  which, 
the  saccus  endolympliaticus,  is  found  in  the  adult  occupying 
the  aqueductus  vestibuli  of  the  temporal  bone. 

The  opposite,  anterior  or  ventral  extremity  of  the  otic 
vesicle  also  bulges  out  into  a  small  evagination,  which  grad- 
ually elongates  until  it  is  a  tapering  tube,  slightly  curved 
inward  toward  the  median  plane.  This  lengthens  still  more 
and  becomes  spirally  coiled,  forming  the  cochlear  duct  or 
scala  media  of  the  future  cochlea  (Fig.  152).  The  vesicle 
itself  becomes  constricted  in  such  manner  by  an  inward  pro- 
jection of  its  wall  as  to  indicate  its  division  into  an  upper 
larger  and  a  lower  smaller  sac,  the  terms  upper  and  lower 
referring  respectively  to  the  head-end  and  the  tail-end  of  the 
embryonic  body.  Before  the  constriction  occurs,  the  wall 
of  that  part  of  the  vesicle  which  is  to  become  the  future 
upper  or  utricular  division  presents  two  pouched-out  areas 
(Fig.  151,  B).  One  of  these  gives  rise  to  the  external  semi- 
circular canal,  while  from  the  other  are  formed  the  superior 
and  posterior  canals.     The  pouch  that  produces  the  external 


Fig.  152.— Diagram  to  illustrate  the  ultimate  condition  of  the  membranous  laby- 
rinth (after  Waldeyer) :  u,  utriculus ;  s,  sacculus ;  cr,  canalis  reuniens ;  r,  ductus 
endolymphaticus ;  c,  cochlea ;  k,  blind  sac  of  the  cupola ;  v,  vestibular  blind  sac 
of  the  ductus  cochlearis. 

canal  is  semicircular  in  form  and  flat,  lying  in  the  horizon- 
tal plane,  its  upper  and  lower  walls  being  in  contact  with 
each  other.  The  opposed  walls  fuse,  except  at  the  periphery 
of  the  pocket,  and  hence  all  that  remains  of  its  cavity  is  a 
small  marginal  tube  or  channel,  corresponding  with  its  bor- 
der and  opening  at  each  end  into  the  cavity  of  the  vesicle. 
Throughout  the  region  of  fusion  of  the  walls,  the  latter  be- 


324  TEXT-BOOK  OF  EMBRYOLOGY. 

come  thin  and  finally  disappear,  being  replaced  by  connec- 
tive tissue.  Thus  a  semicircular  epithelial  tube  is  formed, 
which  is  the  horizontal  or  external  semicircular  canal.  One 
end  of  the  tube  being  dilated,  the  ampulla  of  the  canal  is 
produced. 

The  superior  and  posterior  semicircular  canals  are  formed 
in  a  somewhat  similar  manner  by  the  other  evaginated 
pouch  or  pocket,  which  is  irregularly  globular.  To  pro- 
duce this  result,  the  walls  of  the  pocket  contract  adhe- 
sions throughout  two  regions,  which  correspond  with  the 
respective  spaces  enclosed  by  each  of  the  two  future  canals 
in  question.  The  fusion  of  the  walls  takes  place  in  such 
manner  as  to  leave  two  narrow  channels  or  tubes,  one  of 
which  almost  encircles  the  inner  or  mesial  aspect  of  the 
pocket,  while  the  other  bears  the  same  relation  to  its  poste- 
rior wall,  the  inner  limb  of  the  latter  semicircle  coinciding 
with  the  posterior  limb  of  the  former.  The  result  of  this 
arrangement  is  that  two  vertical  semicircular  canals  are 
formed  with  their  planes  at  right  angles  to  each  other,  the 
two  communicating  with  the  otic  vesicle  by  three  openings, 
one  of  which  is  common  to  both  canals.  The  other  two 
apertures,  being  dilated,  are  the  ampullated  individual  ori- 
fices of  the  posterior  and  superior  canals. 

The  constriction  in  the  otic  vesicle  referred  to  above  in- 
creases until  this  sac  is  divided  into  two  parts,  a  larger, 
which  includes  the  region  from  which  the  semicircular 
canals  have  developed  and  which  is  now  the  utricle,  and  a 
smaller  vesicle,  the  saccule,  comprising  the  part  from  wliich 
the  cochlear  duct  was  evaginated  (Fig.  152).  The  line  of 
division  coincides  with  the  middle  of  the  orifice  of  the  ductus 
endolymphaticus,  the  proximal  end  of  which  participates  in 
the  division.  Thus  the  ductus  endolym])haticus  becomes  a 
Y-shaped  tube,  and  aflFords  the  only  bond  of  connection  be- 
tween the  saccule  and  the  utricle  (Fig.  152). 

The  beginning  of  the  cochlear  duct,  failing  to  keep  pace 
in  growth  with  the  other  parts,  appears  as  a  smaller  tube 
relatively,  and  is  known  as  the  canalis  reuniens  (Fig,  152, 
cr). 


THE  INTERNAL  EAR.  325 

The  structures  so  far  considered — the  utricle,  the  saccule, 
the  semicircular  canals,  and  the  cochlear  duct — being  the  prod- 
uct of  the  ectodermic  otic  vesicle,  represent  simply  the  adult 
epithelial  linings  of  those  cavities.  The  fibrous  layer  of  the 
membranous  labyrinth,  in  common  with  the  walls  of  the  bony 
labyrinth,  is  a  product  of  the  enveloping  mesodermic  tissue. 
While  the  cells  of  the  otic  vesicle  thus  for  the  most  part  con- 
stitute the  walls  of  the  several  sacs  and  canals  of  the  primi- 
tive internal  ear,  some  of  the  cells  specialize  into  neuro-epi- 
thelium.  The  most  marked  specialization  of  this  sort  occurs 
in  the  cochlear  duct,  where  most  of  the  cells  on  that  wall  of 
the  duct  which  may  be  called  its  floor — the  part  correspond- 
ing to  the  future  membrana  basilaris — undergo  such  profound 
modification  in  form  as  to  produce  the  most  highly  special- 
ized neuro-epithelial  cells  anywhere  to  be  found,  the  elements 
that  constitute  the  organ  of  Corti. 

In  the  utricle  and  the  saccule,  as  well  as  in  the  ampullae 
of  the  semicircular  canals,  there  is  a  similar  but  less  marked 
specialization  of  epithelial  cells  to  produce  in  the  former  case 
the  maculse  acusticse,  and  in  the  latter,  the  cristas  acusticse  of 
the  ampullse.  While,  therefore,  the  cells  of  the  otic  vesicle 
which  are  to  serve  as  the  lining  mucous  membrane  of  the 
membranous  labyrinth  become  flattened  polyhedral  cells 
arranged  as  a  single  layer,  those  cells  which  are  to  function- 
ate as  the  peripheral  part  of  the  acoustic  mechanism  become 
the  specially  modified  columnar  cells,  many  of  them  with 
cilium-like  appendages,  of  the  maculse,  the  cristse,  and  of  the 
organ  of  Corti. 

From  the  first  the  otic  vesicle  lies  in  close  relation  with 
the  acusticofacial  ganglion  (Fig.  151,  B).  As  pointed  out 
in  a  preceding  chapter  (p.  297),  this  ganglion  subsequently 
divides  into  two  parts,  corresponding  with  the  two  divisions 
of  the  auditory  nerve.  This  division  of  the  ganglion  and  of 
the  nerve  is  correlated  with  the  separation  of  the  otic  vesicle 
into  a  cochlear  part,  the  cochlear  duct,  and  the  two  vestibular 
vesicles,  the  saccule  and  the  utricle.  While  the  cochlear  duct 
is  still  a  short,  slightly  curved  tube,  the  cochlear  part  of  the 
ganglion  lies  in  close  proximity  to  the  tube,  in  the  concavity 


326  TEXT-BOOK  OF  EMBRYOLOGY. 

ou  its  inner  side.  As  the  duct  lengthens  and  becomes  more 
coiled,  the  ganglion  likewise  lengthens  into  a  band  which 
follows  the  spiral  course  of  the  duct,  lying  parallel  with  the 
latter  and  on  the  side  toward  the  axis  about  which  it  is 
coiled.  After  the  formation  of  the  bony  parts  of  the  cochlea, 
this  ganglion  occupies  the  spiral  canal  of  the  modiolus  and 
is  known  as  the  ganglion  spirale.  It  belongs  to  the  cochlear 
division  of  the  auditory  nerve,  which  is  distributed  to  the 
cochlea. 

The  remaining  part  of  the  acoustic  ganglion  becomes  rather 
widely  separated  from  the  spiral  ganglion,  coming  to  occupy 
a  position  in  the  internal  auditory  meatus,  and  the  part  of  the 
auditory  nerve  with  which  it  is  connected  acquires  relation 
with  the  macular  regions  of  the  utricle  and  saccule  as  well  as 
with  the  cristse  of  the  ampullae  of  the  semicircular  canals. 
These  nerve-fibers  constitute  the  vestibular  division  of  the 
auditory  nerve,  while  the  ganglion  is  the  vestibular  ganglion 
or   intumescentia  ganglioformis  of  Scarpa, 

The  development  of  the  bony  labyrinth  of  the  internal  ear, 
as  well  as  of  the  connective-tissue  parts  of  the  membranous 
labyrinth,  is  eifected  solely  by  the  differentiation  of  the  meso- 
derraic  tissue  which  surrounds  the  epithelial  structures  above 
considered.  As  previously  stated,  at  the  time  M^ien  the  otic 
vesicle  is  first  formed  there  is  no  indication  of  a  cranial  cajD- 
sule,  the  brain-vesicles  being  surrounded  and  separated  from 
the  ectoderm  by  indifferent  mesodermic  cells.  During  the 
progress  of  the  alterations  in  the  otic  vesicle,  this  tissue 
undergoes  condensation  and  alteration  to  form  the  mem- 
branous primordial  cranium,  and  shortly  thereafter  the 
petrous  portion  of  the  temporal  bone  is  outlined  in  cartilage 
by  the  further  specialization  of  a  portion  of  this  primitive 
connective  tissue.  The  formation  of  cartilage  does  not  affect 
all  of  the  tissue  which  is  afterward  represented  by  the 
petrosa,  the  region  that  borders  the  semicircular  canals, 
the  cochlear  duct,  the  saccule,  and  the  utricle  remaining  soft 
embryonal  connective  tissue.  There  is  thus  a  cartilaginous 
ear-capsule  produced  which  is  more  than  large  enougli  to 
contain  the  primitive  epithelial  labyrinth,  and  the  walls  of 


THE  INTERNAL  EAR.  327 

which  are  separated  from  the  latter  by  embryonal  connective 
tissue. 

,  The  bony  semicircular  canals  are  almost  exact  reproduc- 
tions, on  a  larger  scale,  of  the  epithelial  canals,  and  they  are 
formed  by  the  ossification  of  the  cartilaginous  petrosa.  Even 
before  this  ossification  occurs  the  soft  connective  tissue 
between  the  cartilage  and  the  epithelial  semicircular  canals 
differentiates  into  three  layers.  The  inner  layer,  becoming 
more  condensed,  is  converted  into  fibrous  tissue,  and,  adher- 
ing to  the  epithelial  walls  of  the  canals,  furnishes  the  con- 
nective-tissue component  of  the  completed  membranous 
canals.  Its  blood-vessels  serve  for  the  nutrition  of  the 
canals.  The  outer  layer  also  undergoes  condensation  and 
forms  a  fibrovascular  membrane,  the  perichondrium,  which 
later  becomes  the  internal  periosteum  of  the  bony  canals. 
The  middle  layer,  on  the  contrary,  becomes  softer — by  the 
liquefaction  of  the  intercellular  substance  and  the  degenera- 
tion of  the  cells — so  that  gradually  increasing,  fluid-filled 
cavities  make  their  appearance,  and  these  latter  becoming 
larger  and  many  of  them  coalescing,  a  space  is  formed 
around  the  membranous  canals  which  is  filled  with  fluid,  the 
perilymph.  This  perilymphatic  space  is  bridged  across  at 
intervals  by  connective-tissue  processes  that  serve  for  the 
conveyance  of  blood-vessels  to  the  membranous  canals. 

The  vestibule  of  the  internal  ear  is  formed  in  practically 
the  same  manner  as  the  bony  semicircular  canals,  the  epi- 
thelial saccule  and  utricle  acquiring  their  connective-tissue 
constituents  in  the  same  way.  There  is  the  difference,  how- 
ever, that  the  bony  vestibule  does  not  conform  to  the  shape 
of  the  vestibular  parts  of  the  membranous  labyrinth,  since 
it  is  a  single  undivided  cavity  enclosing  the  two  little  ves- 
icles, the  saccule  and  the  utricle. 

The  bony  cochlea,  while  developed  upon  the  same  general 
plan  as  the  other  parts  of  the  bony  labyrinth,  presents  cer- 
tain conspicuous  modifications.  The  epithelial  cochlear  duct, 
as  stated  above,  in  its  early  stage  is  a  short,  tapering,  and 
slightly  curved  tube.  While  it  is  still  in  this  condition, 
chondrification    of  the    petrous   bone    occurs,    whereby    the 


328 


TEXT-BOOK  OF  EMBRYOLOGY. 


duct  acquires  its  cartilaginous  capsule  (Fig.  153,  hh).  This 
capsule  is  open  at  the  proximal  end  of  the  duct  and 
through  this  opening  the  cochlear  branches  of  the  audi- 
tory nerve  gain  access  to  the  capsule,  being  connected  with 
the  cochlear  division  of  the  auditory  ganglion,  which,  owing 
to  its  previously  having  assumed  a  position  beside  the  duct, 
comes  to  be  enclosed  by  the  capsule  as  the  latter  is  formed 
(Fig.  153,  nc,  gsp).  It  is  only  after  the  chondrification 
that  the  cochlear  duct  lengthens  out  and  becomes  spirally 


Fig.  153.— Part  of  a  section  through  the  cochlea  of  an  embryo  cat,  9  cm.  (3.6  in.) 
long  (after  Boettcher) :  kk,  cartilaginous  capsule,  in  which  the  cochlear  duct 
describes  ascending  spiral  turns ;  dc,  ductus  cochlearis ;  c,  organ  of  Corti ;  Iv, 
lamina  vestibularis :  x,  outer  wall  of  the  membranous  ductus  cochlearis  with  liga- 
mentum  spirale;  .SF,  scala  vestibuli;  ST,  ST',  scala  tympani ;  gr,  gelatinous  tissue, 
which  still  fills  the  scala  vestibuli  {sv')  in  its  last  turns ;  g',  remnant  of  the  gela- 
tinous tissue,  which  is  not  yet  liquefied ;  71/,  firm  connective  tissue  surrounding 
the  cochlear  nerve  (nc) ;  gsp,  ganglion  spirale;  N,  nerve  which  runs  to  Corti's 
organ  in  the  future  lamina  spiralis  ossea ;  }',  compact  connective-tissue  layer,which 
becomes  ossified  and  shares  in  bounding  the  bony  cochlear  duct ;  P,  perichon- 
drium. 


coiled.     The  coiling  is   in   such   manner  that  the   cochlear 
nerve  is  surrounded  by  the  duct — that  is,  it  lies  in  the  axis 


THE  INTERNAL   EAR.  329 

about  which  the  duct  is  spirally  wound.  Within  the  carti- 
laginous capsule,  filling  all  the  space  not  occupied  by  the 
spirally  coiled  duct  and  the  cochlear  nerve  with  its  length- 
ened-out  ganglion,  is  the  embryonic  connective  tissue  of 
which  formerly  the  entire  cartilaginous  petrosa  consisted. 
The  cochlea  consists  now  of  a  spirally  coiled  epithelial  tube 
lying  within  an  elongated  cavity  in  the  cartilaginous  petrosa, 
a  cavity,  the  walls  of  which  are,  therefore,  cartilaginous. 
The  peripheral  wall  of  the  coiled  tube  is  in  contact  with  the 
inner  surface  of  the  wall  of  the  cartilaginous  capsule  (Fig. 
153,  x),  a  fact  which  has  an  important  bearing  upon  the 
further  stages  of  growth. 

The  embryonal  connective  tissue  within  the  capsule  now 
undergoes  important  modifications,  which  vary  greatly  in  dif- 
ferent regions.  That  portion  of  this  tissue  which  immediately 
envelopes  the  cochlear  nerve  becomes  first  dense  connective 
tissue,  which  is  afterward  directly  converted  into  bone,  con- 
stituting the  modiolus,  or  axis,  of  the  cochlea.  The  proc- 
esses of  condensation  and  subsequent  ossification  extend 
outward  from  the  modiolus  in  a  spiral  line,  which  corresponds 
with  the  intervals  between  the  successive  turns  of  the  coch- 
lear duct,  until  they  meet  the  wall  of  the  original  capsule, 
thus  producing  the  bony  cochlea.  That  is,  by  the  develop- 
ment of  this  spiral  plate  and  its  connection  internally  with 
the  modiolus  and  externally  with  the  wall  of  the  capsule,  a 
tube  at  first  partly  membranous  and  partly  cartilaginous, 
and  at  a  later  stage  osseous,  is  produced,  which  encloses  the 
much  smaller  cochlear  duct,  and  like  it  is  wound  spirally 
around  the  modiolus.  To  repeat,  the  original  simple  cavity 
of  the  cartilaginous  capsule  is  subdivided  by  the  growth  of 
the  modiolus  and  of  the  spiral  shelf  in  such  manner  as  to 
become  a  spirally  coiled  tube. 

The  cochlear  nerve,  enclosed  within  the  coil  of  the  coch- 
lear duct,  sends  branches  (Fig.  153,  N)  in  a  continuous 
spiral  line  to  the  duct,  and  the  soft  tissue  surrounding  and 
supporting  these  branches  condenses  to  form  a  connective- 
tissue  plate  which  extends  outward  from  the  modiolus  to 
the  cochlear  duct  and  which,  therefore,  has  a  spiral  course 


330  TEXT-BOOK  OF  EMBRYOLOGY. 

about  the  modiolus,  its  entire  inner  edge  being  attached 
to  that  central  axis,  while  its  outer  border  is,  throughout 
its  entire  extent,  in  continuity  with  the  inner  wall  of  the 
duct.  At  a  later  stage  this  spiral  plate  undergoes  direct  os- 
sification to  form  the  two  lamellae  of  the  bony  lamina  spiralis. 
Thus  it  is  that  the  ganglion  spirale  and  tlie  successive  ter- 
minal branches  of  the  cochlear  nerve  come  to  be  enclosed 
within  the  spiral  lamina.  Recalling  the  condition  of  the 
cochlea  before  the  growth  of  the  spiral  lamina,  it  will  be  seen 
that  the  latter,  in  connection  with  the  epithelial  cochlear  duct, 
divides  the  tube  into  two  parts  (Fig.  153,  SV,  ST).  It 
will  be  evident,  too,  that  the  epithelial  cochlear  duct  now 
holds  a  relation  to  the  larger  tube  of  the  future  bony  cochlea 
which  is  similar  in  principle  to  the  relation  of  the  mem- 
branous semicircular  canals  to  the  bony  canals,  but  with 
the  difference  that  the  outer  wall  of  the  epithelial  duct  is  in 
close  contact  with  the  outer  wall  of  the  future  bony  canal 
at  x,  and  that  the  inner  walls  of  the  two  are  connected  by  a 
spiral  plate,  the  lamina  spiralis. 

The  cochlear  duct,  then,  is  surrounded  by  undifferentiated 
mesodermic  tissue,  except  on  the  side  farthest  from  the 
modiolus,  where  its  wall  is  in  contact  with  and  finally 
adheres  to  the  wall  of  the  cartilaginous  capsule.  The  lamina 
spiralis  divides  this  tissue  into  two  parts  which  respectively 
occupy  the  positions  of  the  future  scala  vestibuli  and  scala 
tympani.  This  soft  embryonal  tissue,  as  in  the  case  of  the 
corresponding  tissue  of  the  semicircular  canals,  develops  dif- 
ferently in  different  regions.  The  innermost  stratum,  which  is 
in  relation  with  the  epithelial  cochlear  duct,  becomes  fibrous 
connective  tissue  and  constitutes  the  fibrous  layer  of  the  adult 
cochlear  duct ;  that  is,  on  the  side  of  the  duct  toward  the 
scala  tympani,  it  becomes  the  connective-tissue  layer  of  the 
membrana  basilaris,  while  on  the  side  toward  the  scala  ves- 
tibuli it  forms  the  fibrous  stratum  of  the  membrane  of  Reiss- 
ner  (Fig.  153).  The  peripheral  zone  of  indifferent  tissue, 
that  in  contact  with  the  now  cartilaginous  wall  of  the  future 
bony  cochlea,  as  well  as  that  which  lies  against  the  lamina 
spiralis,  also  undergoes  condensation  and  forms  a  fibrous, 


THE  MIDDLE  AND   THE  EXTERNAL  EAR.  331 

or  fibro vascular,  membrane,  the  internal  perichondrium  or 
future  periosteum.  The  tissue  intervening  between  these 
two  layers  retrogrades,  the  cells  degenerating  and  the  inter- 
cellular substance  liquefying,  until  finally  the  spaces  known  as 
the  scala  vestibuli  and  the  scala  tympani  are  hollowed  out. 
These  channels  are  lymph-spaces  and  the  fluid  they  contain 
is  the  perilymph.  This  perilymphatic  space  is  in  communi- 
cation with  that  of  the  vestibule.  Therefore,  while  the  coch- 
lear duct  or  scala  media  encloses  an  epithelium-lined  space, 
as  do  the  saccule,  the  utricle,  and  the  membranous  semi- 
circular canals,  and  in  common  with  those  structures  con- 
tains the  so-called  endolymph,  the  scala  vestibuli  and  the 
scala  tympani  are  in  the  same  category  with  the  perilym- 
phatic spaces  of  the  other  parts  of  the  internal  ear. 

The  Middle  and  the  i^xternal  Bar. — The  middle  ear, 
consisting  of  the  tympanic  cavity  and  the  Eustachian  tube, 
is  developed  from  the  back  part  or  dorsal  end  of  the  first 
inner  visceral  furrow.  The  external  ear,  comprising  the  ex- 
ternal auditory  meatus  and  the  auricle,  comes  from  the  dor- 
sal extremity  of  the  first  outer  furrow  and  the  tissue  about 
its  margins,  the  tympanic  membrane  representing  in  part  the 
closing  membrane  which  separates  the  inner  furrow  from  the 
outer. 

The  first  inner  visceral  furrow,  in  common  with  the 
other  inner  furrows,  is  an  evagination  of  the  lateral  wall 
of  the  primitive  pharyngeal  cavity,  or  head-end  of  the  gut- 
tract.  The  ventral  end  of  this  groove  suffers  obliteration, 
but  the  dorsal  segment,  designated  the  tubotympanic  sul- 
cus, becomes  converted  into  a  tube  by  the  growing  together 
of  its  edges.  The  tube  is  composed  therefore  of  entodermic 
epithelial  cells.  It  elongates  in  the  dorsal  and  outward 
direction,  and  its  dorsal  extremity  becomes  enlarged  to  pro- 
duce the  cavity  of  the  tympanum,  the  remaining  part  of  the 
canal  becoming  the  epithelial  lining  of  the  Eustachian  tube. 
The  canal  being  formed  before  the  development  of  the 
cranium,  and  approximately  its  posterior  half  being  sur- 
rounded by  the  mesodermic  embryonal  connective  tissue 
that  afterward  becomes  the  petrosa  of  the  temporal  bone,  the 


iiS'2  TEXT-BOOK  OF  EMBRYOLOGY. 

tympanic  cavity  and  a  part  of  the  Eustachian  tube  come  to 
be  enclosed  within  that  bone,  while  the  connective  tissue 
encasing  the  anterior  part  of  the  tube  diiferentiates  into  the 
curved  plate  of  cartilage  that  forms  the  cartilaginous  part  of 
the  Eustachian  tube. 

Since  the  posterior  end  of  the  primitive  epithelial  tube 
insinuates  itself  between  the  otic  vesicle  and  the  surface, 
the  tympanum  comes  to  occupy  its  normal  position  on  the 
outer  side  of  the  internal  ear.  The  tympanum,  being  de- 
rived from  the  back  part  of  the  first  visceral  cleft,  is  in 
close  relation  with  the  first  and  second  visceral  arches,  and 
the  ossicles  of  the  middle  ear  are  derived  from  the  dorsal 
extremities  of  the  cartilaginous  bars  of  these  arches  in  the 
manner  described  in  Chapter  XYIII.  Necessarily  the 
primitive  ossicles  are  exterior  to  the  primitive  epithelial 
tympanic  sac,  as  is  also  the  chorda  tympani  nerve,  which 
passes  along  its  outer  side.  After  the  ossification  of  the 
temporal  bone,  these  structures  are  embedded  within  the 
abundant  soft  connective  tissue  which  is  between  the  epi- 
thelial sac,  now  the  mucous  membrane,  and  the  bony  walls 
of  the  tympanum.  This  mass  of  soft  tissue  undergoes  very 
considerable  diminution,  owing  to  which  the  mucous  mem- 
brane comes  into  contact  with  the  bony  Avails,  and  as  a  result 
the  ossicles  and  the  chorda  tympani  are  enclosed  in  folds  of 
the  mucous  membrane  and  seem  to  lie  within  the  tympanic 
cavity.  They  are  excluded,  however,  from  the  true  cavity 
of  the  tympanum,  since  they  are  exterior  to  the  epithelial  or 
mucous-membrane  layer. 

The  external  auditory  meatus  is  simply  the  persistent  pos- 
terior part  of  the  first  outer  visceral  furrow  or  hyoman- 
dibular  cleft  (see  pp.  101,  105),  this  cleft  closing  completely 
everywhere  but  in  this  region.  The  closing  plate  of  the  first 
cleft  becomes  the  tympanic  membrane.  Hence  the  outer 
layer  of  this  membrane  is  of  ectodermic  origin,  while  the 
inner  layer  is  entoderraic,  being  continuous  with  the  epi- 
thelial tympanic  lining,  and  the  middle  fibrous  layer  is 
derived  from  the  mesoderm.  The  relation  of  the  malleus  to 
the  membrane  and  of  the  latter  to  the  bony  tympanic  plate 


THE  INTERNAL  EAR. 


333 


which  forms  part  of  the  wall  of  the  meatus  is  dealt  with  in 
the  chapter  on  the  development  of  the  skeleton. 

The  auricle  is  derived  from  the  tissue  around  the  margin 
of  the  unclosed  back  part  of  the  first  outer  cleft  (Fig.  155,  C). 
Six  little  elevations  make  their  appearance  here,  the  projections 
being  mesodermic  tissue  covered  with  ectoderm.  The  meso- 
dermic  component  of  the  elevations  differentiates  into  the 
cartilaginous  and  other  connective-tissue  parts  of  the  auricle. 
The  nodules  marked  2  and  3  in  Fig.  154  becoming  a  continu- 


FiG.  154.— Showing  the  gradual  development  of  the  parts  of  the  external  ear 
from  prominences  upon  the  mandibular  and  hyoidean  visceral  arches  (His),  vari- 
ously magnified :  1,  2,  prominences  on  mandibular  arch  ;  3,  prominence  between 
the  two  arches,  prolonged  posteriorly  in  second  figure  to  3c;  4,5,  and  6,  promi- 
nences on  hyoidean  or  second  visceral  arch ;  K,  lower  jaw.  Prominence  1  forms 
the  tragus  ;  2,  3,  3c,  the  helix ;  4,  the  antihelix  ;  5,  the  antitragus  ;  6,  the  lobule. 

ous  ridge,  produce  the  helix,  while  nodule  4  becomes  the  anti- 
helix.  The  tragus  and  antitragus  develop  respectively  from 
the  projections  1  and  5.  At  the  end  of  the  second  month, 
these  parts  are  so  far  advanced  as  to  be  easily  distinguish- 
able, and  the  connective-tissue  basis  of  the  ridges  and  pro- 
jections and  the  continuous  plate-like  mass  to  which  they 
all  are  attached  begin  to  undergo  chondrification.  From 
the  third  month  onward,  this  primitive  auricle,  by  continued 
growth  and  greater  separation  from  the  side  of  the  head, 
assumes  more  and  more  the  characters  of  the  fully  formed 
member.  The  lobule,  however,  which  results  from  the 
growth  of  the  little  elevation  marked  6,  lags  behind  the 
other  parts  in  development  and  is  rather  indistinct  until  the 


334  TEXT-BOOK  OF  EMBRYOLOGY. 

fifth  month,  after  which  time  it  increases  in  size  and  gradu- 
ally acquires  its  normal  proportions. 


THE  DEVELOPMENT  OF  THE  NOSE. 

The  nose  is  primarily  a  special  sense-organ,  although  a 
part  of  its  cavity  serves,  in  air-breathing  vertebrates,  as  an 
adjunct  to  the  respiratory  system.  The  evolution  of  the 
mature  organ  of  smell  may  be  epitomized  by  the  statement 
that  the  olfactory  epithelium,  the  essential  part  of  this  sense- 
organ,  is  a  patch  of  depressed  or  infolded  ectoderm,  the  cells 
of  which  are  highly  specialized  and  are  brought  into  relation 
with  the  central  nervous  system  by  means  of  the  outgrowth 
from  the  latter  of  a  part  of  its  mass,  the  olfactory  lobe. 

Very  early  in  intra-uterine  life — before  the  end  of  the 
third  week — the  olfactory  plates  appear  as  localized  thicken- 
ings of  the  ectoderm  situated  just  in  front  of  or  above  the 
oral  fossa.  These  nasal  areas  are  the  forerunners  of  the 
future  olfactory  epithelium.  It  is  worthy  of  note  that  the 
olfactory  plates  are  in  very  close  relation  with  the  primary 
fore-brain  vesicle,  being,  in  reality,  on  the  outer  surface  of 
the  ectodermic  covering  of  its  ventral  wall. 

Owing  to  the  rapid  outgrowth  of  the  surrounding  tissue, 
the  olfactory  plates  become  relatively  depressed,  constituting 
now  the  nasal  pits,  which  are  distinguishable  at  about  the 
twenty-eighth  day.  The  pits  are  separated  from  each  other  by 
a  broad  mass  of  tissue,  the  nasal  or  nasofrontal  process  (Fig. 
155),  which  is,  as  it  were,  a  localized  thickening  of  the  meso- 
dermic  tissue  on  the  ventral  wall  of  the  primary  fore-brain  ves- 
icle ;  and  this  process  makes  its  appearance  in  the  third  week. 
During  the  fifth  week  the  nasofrontal  process  thickens  greatly 
along  its  lateral  margins,  the  thick  edges  being  known  as  the 
globular  processes  (Fig.  155,  A,  B).  At  the  same  time  the 
lateral  nasal  processes  bud  out  from  the  nasofrontal  process, 
one  on  each  side,  above  the  nasal  pits,  and,  growing  down- 
ward, form  the  external  boundaries  of  the  pits,  each  of  which 
depressions  is  bounded  on  its  inner  side  by  the  corresponding 
globular  process.    The  nasal  pits,  therefore,  have  well-marked 


^HE  DEVELOPMENT  OF  THE  NOSE. 


335 


walls  on  every  side  except  below,  where  they  are  directly 
continuous  with  the  oral  fossa. 

In  the  latter  end  of  the  sixth  week  the  nasofrontal  proc- 
ess, which,  it  will  be    remembered,   constitutes   the  upper 


Tig.  155.— Development  of  the  face  of  the  human  embryo  (His) :  A,  embryo  of 
about  twenty-nine  days.  The  nasofrontal  plate  differentiating  into  processus 
globulares,  toward  which  the  maxillary  processes  of  first  visceral  arch  are  extend- 
ing. B,  embryo  of  about  thirty-four  days  :  the  globular,  lateral  frontal,  and  max- 
illary processes  are  in  apposition  ;  the  primitive  opening  is  now  better  defined.  C, 
embryo  of  about  the  eighth  week:  immediate  boundaries  of  mouth  are  more  defi- 
nite and  the  nasal  orifices  are  partly  formed,  external  ear  appearing.  D,  embryo 
at  end  of  second  month. 

limit  of  the  oral  fossa,  is  joined  on  each  side  by  the  united 
maxillary,  and  lateral  nasal,  processes.  This  effects  a  divis- 
ion between  the  oral  fossa  and  the  nasal  pits,  and  forms, 


336 


TEXT-BOOK  OF  EMBRYOLOGY. 


though  as  yet  crudely,  the  external  nose,  and  the  upper  lip 
as  well.  The  definite  formation  of  the  external  nose  may  be 
said  to  be  indicated  about  the  eighth  week.  The  orifices  of 
the  nasal  pits  are  now  the  anterior  nares,  while  the  pits  them- 
selves have  become  short  canals,  opening  by  their  deep 
orifices,  the  posterior  nares,  into  the  primitive  mouth-cavity 
above  the  palatal  shelves.  The  nares  are  separated  from 
each  other  by  the  still  broad  nasofrontal  process.  That 
portion  of  the  nasofrontal  process  that  separates  the  nares 
gradually  becomes  thinner  and  produces  the  septum  of  the 
nose,  while  its  external  or  superficial  part  gives  rise  to  the 
bridge  and  tip  of  the  organ. 

The  growth  of  the  palate-shelves  (Fig.  156)  toward  the 
median  line,  resulting  in  their  union  with  each  other  and 


Fig.  156.— Roof  of  the  oral  cavity  of  a  human  embryo  with  the  fundaments  of  the 
palatal  processes  (after  His),  X  10. 

with  the  recently-formed  septum,  definitely  divides  the  nasal 
chambers  from  the  cavity  of  the  mouth,  the  posterior  nares 
now  opening  into  the  pharynx.  This  separation  is  completed 
toward  the  end  of  the  third  month. 

The  complexity  of  the  adult  nasal  cavities  is  produced  by 
the  formation  of  ridges  and  pouches  on  the  lateral  walls  of 
the  original  nasal  pits.  Three  inwardly  projecting  horizontal 
folds  of  the  ectoderm ic  lining  of  the  cavity,  the  superior,  mid- 
dle, and  inferior  turbinal  folds,  appear  upon  the  outer  wall  of 
each  nasal  fossa  (Fig.  1 57).     Each  fold  contains  a  stratum 


THE  DEVELOPMENT  OF  THE  NOSE.  337 

of  mesodermic  tissue  which  develops  into  cartilage  and  sub- 
sequently into  bone,  forming  respectively  the  three  turbinated 
bones.  The  cartilaginous  character  of  these  folds  becomes 
apparent  at  the  end  of  the  second,  or  the  early  part  of  the 
third,  month.     An  evagination  on  the  lateral  wall  of  each 


Fig.  157.— Cross-section  through  the  head  of  an  embryo  pig  o  cm.  (1.2  in.)  long, 
crown-rump  measurement.  The  nasal  cavities  are  seen  to  be  iu  communication 
with  the  oral  cavity  at  the  places  designated  by  a  * :  K,  cartilage  of  the  nasal  sep- 
tum ;  m,  turbinal  cartilage ;  J,  organ  of  Jacobson ;  /',  the  place  where  it  opens  into 
the  nasal  cavity ;  gf,  palatal  process ;  of,  maxillary  process ;  zl,  dental  ridge 
(Hertwig). 

nasal  fossa,  between  the  middle  and  the  inferior  turbinal  proc- 
esses, becomes  the  antrum  of  Highmore ;  this  is  formed  in 
the  sixth  month.  Other  evaginations  produce  the  etlunoidal,, 
the  frontal,  and  the  sphenoidal  sinuses,  the  last  two  of  which 
are  not  completed,  however,  until  after  birth.  Very  early  in. 
the  development  of  the  nose  a  small  invagination  appears  on 
the  mesial  wall  of  the  nasal  pit.  In  the  fourth  month  of 
gestation  this  invagination  has  become  a  canal  in  the  septum 
(Fig.  157,  J),  running  from  before  backward  and  ending  in 
a  blind  extremity.  It  is  the  so-called  organ  of  Jacobson, 
which,  in  man,  is  merely  a  rudimentary  structure,  but  which, 
in  most  other  mammals,  is  more  highly  developed,  being 
surrounded  by  a  cartilaginous  capsule  and  receiving  a  special 
nerve-supply  from  the  olfactory  nerve. 

The  olfactory  plates  become  separated  from  the  fore-brain 
vesicle  and  consequently  from  the  later  brain  and  its  out- 

22 


338  TEXT-BOOK  OF  EMBRYOLOGY. 

growth,  the  olfactory  bulb,  by  the  development  of  an  inter- 
vening bony  plate,  the  cribriform  lamina  of  the  ethmoid 
bone.  The  ectodermic  cells  of  the  olfactory  plates  diifereuti- 
ate  into  the  highly  specialized  neuro-epithelial  elements  of  the 
olfactory  mucous  membrane  and  their  associated  supporting 
cells.  The  neurits  of  the  neuro-epithelial  cells,  assuming  re- 
lationship with  the  glomeruli  of  the  olfactory  bulb,  constitute 
the  olfactory  nerve-fibers. 

The  external  nose,  as  previously  stated,  first  acquires  defi- 
nite form  about  the  eighth  week  by  the  union  of  the  distal 
ends  of  the  lateral  nasal  processes  with  the  nasofrontal  proc- 
ess, the  former  producing  the  alae  and  the  latter  the  bridge 
and  the  tip  of  the  nose.  In  the  third  month  the  organ  is 
unduly  flat  and  broad,  but  from  this  time  on  it  gradually 
assumes  the  familiar  characteristic  form.  From  the  third 
month  to '  the  fifth  each  external  naris  is  closed  by  a  gelat- 
inous plug  of  epithelial  cells. 


CHAPTER   Xyil. 
THE    DEVELOPMENT  OF  THE  MUSCULAR  SYSTEM. 

THE  STRIATED  OR  VOLUNTARY  MUSCLES. 

The  voluntary  muscular  system,  genetically  considered,  is 
divisible  into  (1)  the  muscles  of  the  trunk  and  (2)  those  of  the 
extremities.  The  muscles  of  the  trunk  include  two  distinct 
sets :  (a)  the  muscles  of  the  trunk  proper,  or  the  skeletal 
muscles,  and  (b)  the  muscles  of  the  visceral  arches  or  the 
branchial  muscles. 

To  arrive  at  a  proper  comprehension  of  the  evolution  of 
the  muscular  system  it  is  necessary  to  revert  to  an  important 
fundamental  embryological  process,  the  segmentation  of  the 
body  of  the  embryo,  or,  as  it  is  sometimes  expressed,  the  seg- 
mentation of  the  coelom,  or  body-cavity.  As  pointed  out  in 
Chapter  IV.,  this  process  of  segmentation  occurs  in  all  ver- 
tebrate animals  and  in  some  invertebrates. 

The  Muscles  of  the  Trunk  Proper. — At  a  very  early 
stage  of  development  the  tracts  of  mesodermic  tissue  situated 
one  on  each  side  of  the  median  long-itudinal  axis  of  the  future 
embryonic  body,  the  paraxial  mesodermic  tracts,  undergo 
division  or  segmentation,  in  lines  transverse  to  the  long 
axis,  into  a  series  of  pairs  of  irregularly  cubical  masses 
of  mesodermic  cells.  These  masses  are  the  mesoblastic 
somites  or  primitive  segments,  often  inappropriately  called 
the  proto vertebrae.  The  somite  first  formed  corresponds 
with  the  future  occipital  region,  the  second  one  lies  immedi- 
ately in  front  of  the  first,  while  two  others,  situated  still 
more  anteriorly,  that  is,  near  the  cephalic  end  of  the  embry- 
onic area,  and  seven  more,  behind  the  first,  are  added  almost 
simultaneously.     The  formation  of  the  primitive  segments 

339 


340 


TEXT-BOOK  OF  EMBRYOLOGY. 


then  proceeds  tailward  until  a  considerable  number  have 
been  added.  Those  in  front  of  the  one  first  formed  are 
denominated  the  head-segments,  while  the  others  are  known 
as  the  trunk-segments.  Each  somite  is  at  first  triangular  in 
cross-section,  the  base  of  the  triangle  looking  toward  the 
chorda  dorsalis.  Subsequently  they  assume  a  more  cuboidal 
shape.  In  the  lower  vertebrates — amphibians  and  fishes — 
the  somite  is  hollow,  its  cavity  being  in  these  cases  a  con- 
stricted-off  portion  of  the  body -cavity  (hence  the  term  "  seg- 


FiG.  158.— Cross-section  through  the  region  of  the  pronephros  of  a  selachian 
embryo  in  which  the  muscle-segments  (myotomes)  (mp)  are  in  process  of  being 
constricted  off.  Diagram  (after  Wijhe) :  nr,  neural  tube ;  eh,  chorda ;  ao,  aorta ; 
sch,  subnotochordal  rod ;  mp,  muscle-plate  of  the  primitive  segment ;  w,  zone  of 
growth  where  the  muscle-plate  bends  around  into  the  cutis-plate  (cp) ;  vh,  tract 
connecting  the  primitive  segment  with  the  body-cavity,  out  of  which  are  devel- 
oped, among  other  things,  the  mesonephric  tubules  ;  sk,  skeletogenous  tissue 
which  arises  by  a  proliferation  from  the  median  wall  of  the  connecting  tract  vh; 
vn,  pronephros;  mfc',  mk",  parietal  and  visceral  middle  layer,  from  whose  walls 
mesenchyme  is  developed  ;   Ih,  body-cavity ;  ik,  entoblast. 

mentation  of  the  Cfjelom  "  to  express  this  process).  In  the 
higher  vertebrates,  however,  the  cavity  is  obliterated  by  the 
encroachment  of  the  cells  of  the  walls  of  the  somite. 

The  cells  of  the  somites  soon  undergo  differentiation  and 
rearrangement.     It  is  usually  stated  that,  preparatory  to  the 


THE  STRIATED   OR    VOLUNTARY  MUSCLES.        341 

segmentation  of  the  paraxial  mesodermic  tract,  this  tract  has 
become  separated  from  the  remaining  lateral  plate  of  the 
mesoderm.  The  separation  is  not  complete,  however,  and 
therefore,  after  the  appearance  of  the  primitive  segments, 
each  segment  is  connected  with  the  more  laterally  placed 
lateral  plate — by  the  separation  of  which  latter  into  two 
lamellae  the  coelom  is  formed — by  a  smaller  mass  of  tissue, 
the  nephrotome,  also  called  the  middle  plate,  or  intermediate 
cell-mass  (Fig.  158,  vb).  As  development  progresses  the  dis- 
tinction between  the  primitive  segment  proper  and  the  neph- 
rotome becomes  more  sharply  expressed,  and  the  former  is 
designated  the  myotome.  The  primitive  segment  on  its  me- 
sial surface,  near  the  point  of  union  with  the  nephrotome, 
sends  forth  cells  w^hich  form  a  mass  called  the  sclerotome 
(Fig.  158,  sh).  The  sclerotomes  spread  out  and  blend  with 
each  other,  forming  a  continuous  mass  of  tissue  which  envel- 
ops the  chorda  and  the  neural  canal,  and  which,  being  con- 
cerned in  the  production  of  the  permanent  vertebrae,  has  no 
further  interest  in  this  connection. 

What  remains  of  the  primitive  segment  after  the  forma- 
tion of  the  nephrotome  and  of  the  sclerotome  is  the  myotome 
proper  or  the  muscle-plate.  Although,  as  previously  stated, 
the  primitive  segments  of  the  higher  vertebrates  contain  no 
cavity,  the  myotome  and  the  nephrotome  each  enclose  a 
space,  that  belonging  to  the  former  being  known  as  the 
myocoel.  The  myotomes  or  muscle-plates  are  so  called  be- 
cause they  give  rise  to  the  voluntary  musculature  of  the 
trunk.  But  not  all  of  the  cells  of  the  muscle-plate  undergo  X. 
transformation  into  muscular  tissue.  While  the  cells  on  the 
mesial  or  chordal  side  of  the  myocoel  are  going  through 
certain  alterations  preparatory  to  their  metamorphosis,  the 
cells  nearer  the  body- wall  become  rearranged  to  form  a 
characteristic  layer  which  is  known  as  the  cutis-plate  from 
the  fact  that  it  contributes  to  the  formation  of  the  corium  of 
the  skin  (Fig.  158,  cp).  The  cutis-plate  and  the  remaining 
part  of  the  muscle-plate  are  continuous  around  the  myocoel, 
the  transition  from  one  to  the  other  being  more  or  less  grad- 
ual.    To  summarize,  the  primitive  segment  is  differentiated 


X 


342  TEXT-BOOK  OF  EMBRYOLOGY. 

into  the  nephrotome,  the  sclerotome,  the  myotome  or  muscle- 
plate,  and  the  cutis-plate. 

The  Metamorphosis  of  the  Muscle-plate. — By  the 
term  muscle-plate  is  meant  here  the  thickened  layer  of  cells 
on  the  chordal  or  mesial  side  of  the  myotome  proper,  which 
layer  constitutes  what  remains  of  the  myotome  after  the 
differentiation  of  the  cutis-plate.  These  cells  having  pro- 
liferated and  increased  in  size,  and  having  encroached 
thereby  upon  the  cavity  of  the  myotome,  next  undergo 
alteration  in  shape,  becoming  cylindrical,  with  their  long  axes 
parallel  with  that  of  the  body  of  the  embryo.  The  length 
of  each  cylindrical  cell  equals  the  thickness  of  the  prim- 
itive segment,  at  least  in  the  Amphibia  and  probably  also 
in  the  chick.  The  next  step  in  the  transformation  is  the 
acquisition  of  the  transverse  striation  characteristic  of  ver- 
tebrate voluntary  muscle.  Soon  after  this  the  protoplasm 
of  the  cell  undergoes  longitudinal  division  into  minute 
fibrillse — which  latter  do  not  necessarily  correspond,  how- 
ever, with  the  primitive  fibrillse  of  mature  muscle — and  the 
cell-nucleus  likewise  divides.  The  metamorphosis  of  the 
now  fibrillated  protoplasm  into  muscular  tissue  is  first  com- 
pleted at  the  periphery  of  the  fiber,  so  that  a  young  muscle- 
fiber  contains  a  central  core  of  undifferentiated  material, 
including  the  daughter-nuclei  resulting  from  the  division 
of  the  original  nucleus.  Soon  after  the  appearance  of  stria- 
tion and  the  fibrillation  of  the  fiber,  the  fibers- begin  to  sepa- 
rate from  each  other,  and  developing  connective  tissue  with 
young  blood-vessels  penetrates  between  them,  the  fibers  now 
showing  aggregation  into  bundles.  For  some  time  longer 
the  fibers  are  naked,  since  the  sarcolemma  is  not  acquired 
until  considerably  later.  The  differentiation  into  muscular 
tissue  gradually  extends  from  the  periphery  of  the  fiber  to 
its  core,  the  process  being  complete  in  the  human  embryo  at 
about  the  end  of  the  fifth  month  for  the  muscles  of  the  upper 
extremities  and  in  the  seventh  month  for  those  of  the  lower. 
The  embryonic  muscle-fibers  are  smaller  than  the  mature 
elements  and  increase  in  size  until  the  third  month. 

It  is  considered  highly  probable  by  most  embryologists 


THE  STRIATED   OR   VOLUNTARY  MUSCLES.        343 

that  muscle-fibers  undergo  multiplication  during  embryonic 
life.  There  are  several  theories  as  to  the  method  of  this 
multiplication.  The  most  generally  accepted  view  is  that 
put  forth  by  Weismann,  the  essential  feature  of  which  is 
that  the  fibers  multiply  by  longitudinal  division  or  fission. 
Reference  was  made  above  to  the  repeated  division  of  the 
nucleus  of  the  cell  as  one  of  the  initiatory  steps  in  the  forma- 
tion of  the  muscle-fiber.  According  to  the  fission  theory, 
there  is  one  class  of  fibers  in  which  the  nuclei  are  arranged 
in  a  single  row,  and  the  fibers  of  this  class  do  not  undergo 
fission  ;  while  there  is  another  class,  the  fibers  of  which  have 
their  nuclei  arranged  in  several  rows.  Fibers  of  the  latter 
type  divide  longitudinally  into  as  many  daughter-fibers  as 
there  are  rows  of  nuclei. 

Although  many  of  the  details  of  the  development  of  the 
muscular  system  are  still  involved  in  obscurity,  it  is  a  gen- 
erally accepted  fact  that  each  fiber  is  derived  from  a  single 
cell,  the  protoplasm  of  which  develops  the  function  of  con- 
tractility to  the  subordination  of  the  remaining  vital  proper- 
ties of  protoplasm.  With  this  specialization  of  function 
there  is  necessarily  a  concomitant  alteration  of  structure. 

The  muscular  mass  resulting  from  the  transformation  of 
each  myotome  grows  in  the  ventral  direction  between  the  ecto- 
derm and  the  parietal  leaf  of  the  mesoderm,  or  in  other  words 
into  the  somatopleure,  to  produce  the  muscular  structures  of 
the  ventrolateral  body- wall.  It  grows  also  and  to  a  greater 
extent  in  the  dorsal  direction,  covering,  and  acquiring  points 
of  attachment  to,  the  vertebral  column,  which  has  mean- 
while been  forming.  In  addition  to  the  ventral  and  dorsal 
extension  of  the  muscle-plates,  each  one  grows  both  forward 
and  backward — cephalad  and  caudad — in  such  manner  that 
overlapping  and  intermingling  result. 

What  has  been  said  above  concerning  the  evolution  of 
the  trunk-musculature  from  the  primitive  segments  refers  to 
those  muscles  that  are  developed  from  the  segments  of  the 
trunk.  As  to  the  evolution  of  the  head-segments  compara- 
tively little  is  definitely  known.  It  is  generally  accepted 
that  in  elasmobranchs — a  group  including  sharks  and  rays — 


344  TEXT-BOOK   OF  EMBRYOLOGY. 

there  are  nine  primitive  segments  in  the  region  of  the  future 
head.  The  number  present  in  mammalian  embryos  has  not 
been  clearly  worked  out.  In  the  lower  vertebrates  each 
segment  contains  a  cavity  lined  with  flattened  cells,  the 
mesothelium,  the  metamorphosis  of  which  into  muscular 
tissue  may  be  inferred  to  be  essentially  as  already  outlined 
above.  The  first  head-segment,  which  lies  in  contact  with 
and  partially  envelops  the  optic  vesicle,  gives  rise  to  the  su- 
perior rectus,  the  inferior  rectus,  and  the  inferior  oblique 
muscles  of  the  eyeball ;  the  second  segment  produces  the 
superior  oblique,  and  the  third,  the  external  rectus.  The 
fourth,  fifth,  and  sixth  segments  abort  and  hence  produce  no 
adult  structures ;  while  the  seventh,  the  eighth,  and  the  ninth 
segments  become  metamorphosed  into  the  muscles  that  con- 
nect the  skull  with  the  shoulder-girdle. 

From  recent  studies  ^  it  would  appear  that  individual  mus- 
cles undergo  peculiar  and  significant  migrations  during  their 
development,  and  that  the  origin  of  the  nerve-supply  of  a 
muscle  indicates  the  location  of  the  particular  myotome  or 
myotomes  from  which  it  originated,  since  the  segmental 
nerves  are  connected  with  their  respective  myotomes  and 
supply  the  muscles  derived  from  such  myotomes.  For  ex- 
ample, the  serratus  magnus,  being  innervated  by  branches 
of  the  cervical  nerves,  develops  from  myotomes  in  the  neck 
region,  and  subsequently  moves  down  to  become  attached  to 
the  scapula  and  the  ribs. 

The  Branchial  Muscles. — This  term  embraces  the 
muscles  of  mastication  and  the  various  muscles  connected 
with  the  hyoid  bone,  with  the  jaws,  and  with  the  ossicles  of 
the  middle  ear.  They  result  from  the  metamorphosis  of  the 
mesothelium  of  the  visceral  arches  and  acquire  connections 
with  structures  that  have  arisen  from  the  so-called  mesen- 
chymal cells  of  these  arches  or,  in  other  words,  from  the 
embryonal  connective  tissue  which  makes  up  the  chief  part 
of  their  bulk.  For  an  account  of  the  growth  of  the  visceral 
arches  the  reader  is  referred  to  Chapter  VII.     From  this 

1  See  "Development  of  the  Ventral  Abdominal  Walls  in  Man,"  Frank- 
lin P.  Mall,  Johns  Hopkins  Papers,  vol.  iii.,  1898, 


THE  STRIATED   OR    VOLUNTARY  MUSCLES.        345 

account  and  from  that  found  in  Cha|3ter  IV.,  it  will  be  seen 
that  the  formation  of  the  visceral  arches  and  clefts  is  in 
reality  the  segmentation  of  the  ventral  mesoderm  of  the  head- 
region  of  the  embryo,  or  to  express  it  in  another  way,  it  is 
the  segmentation  of  the  ventral  coelom  of  that  region.  It  is 
interesting  to  note  that  whereas  in  the  trunk  the  segmenta- 
tion of  the  mesoderm  is  restricted  to  the  dorsal  part  of  the 
body,  in  the  head-region  the  ventral  mesoderm  also  partici- 
pates in  the  process.  Hence  the  visceral  arches,  as  might  be 
expected,  consist  of  so  many  masses  of  mesodermic  tissue, 
each  arch  containing  a  small  cavity  lined  with  mesothelium, 
which  cavity  is  a  constricted-oiF  part  of  the  body-cavity  or 
coelom.  It  is  these  mesothelial  cells  that  produce,  by  their 
differentiation,  the  muscles  under  consideration.  While  so 
much  concerning  the  origin  of  this  group  of  muscles  is  jDrac- 
tically  assured  by  observations  upon  the  embryos  of  the 
lower  vertebrates,  the  details  are  still  obscure.  His  assumes 
the  origin  of  the  palatoglossus,  the  styloglossus,  and  the  levator 
palati  froni  the  second  or  hyoid  arch  ;  of  the  stylopharyngeus, 
perhaps  the  palatopharyngeus,  the  hyoglossus  and  the  superior 
constrictor  of  the  pharynx  from  the  third  arch  ;  and  of  the 
middle  and  inferior  pharyngeal  constrictors  from  the  fourth 
arch.  Further,  it  is  held  by  Rabl  that  the  muscles  of  the 
face,  including  those  of  the  scalp  and  the  platysma — the 
muscles  of  expression — originate  from  the  mesothelium  of 
the  hyoid  arch  in  the  form  of  a  thin  superficial  sheet,  which, 
gradually  spreading  out  from  the  place  of  origin,  breaks  up 
into  the  individual  mviscles. 

The  Muscles  of  the  Extremities.— Of  the  develop- 
ment of  these  there  is  little  to  be  said.  Enough  is  known 
of  the  development  of  the  limb-muscles  to  establish  two  im- 
portant facts  :  that  these  muscles  develop  as  buddings  from 
the  muscle-plates  of  the  trunk,  and  that  the  muscles  of  each 
extremity  arise  not  from  one  but  from  several  myotomes, 
the  exact  number  being  uncertain.  The  observations  have 
been  chiefly  upon  selachians  (sharks,  etc.),  and  in  these  em- 
bryos  each  myotome   gives  off  two  buds,  each  of  which 


346  TEXT-BOOK  OF  EMBRYOLOGY. 

divides  into  two  others,  while  the  main  part  of  the  myotome 
continues  its  growth  into  the  somatopleure. 

THE  INVOLUNTARY  OR  UNSTRIATED  MUSCULAR  TISSUE. 

This  variety  of  muscnlar  tissue,  like  that  considered 
above,  is  of  mesodermic  origin.  But  while  the  voluntary 
muscles  arise  from  the  flattened  or  mesothelial  cells  of  the 
primitive  segments,  involuntary  muscle  results  from  the 
transformation  of  the  embryonal  connective-tissue  elements, 
the  mesenchymal  cells,  of  the  mesoderm.  It  is  for  this 
reason  that  some  authors  speak  of  the  voluntary  muscles  as 
the  mesothelial  muscles  and  designate  the  involuntary  mus- 
cular tissue  as  mesenchymal  muscle. 

While  it  is  a  generally  accepted  fact  that  each  of  the  fiber- 
cells  which  make  up  unstriated  muscle  is  a  metamorphosed 
mesenchymal  or  connective-tissue  cell,  the  details  of  the 
process  have  not  been  accurately  worked  out.  One  may 
assume  that  necessarily  the  young  connective-tissue  cell 
elongates  and  that  its  protoplasm  must  undergo  such  differ- 
entiation as  will  fit  it  for  the  exercise  of  its  future  function, 
contractility. 

THE  CARDIAC  MUSCLE. 

-    The  account  of  the  development  of  the  heart-muscle  will 
be  found  in  Chapter  X. 


CHAPTER    XVIII. 

THE  DEVELOPMENT  OF    THE  SKELETON  AND 
OF  THE   LIMB5. 

Although  the  skeleton  is  the  framework  of  the  body  in 
the  anatomical  or  mechanical  sense,  it  is  not  so  embryologic- 
ally,  since  its  development  is  not  begun,  at  least  not  to  any 
important  extent,  until  nearly  all  the  principal  organs  are 
well  diiFerentiated,  and  its  growth  is  largely  subsidiary  to 
that  of  the  structures  which,  in  the  mature  state,  it  supports 
and  protects.  Morphologists  speak  of  the  exoskeleton  and 
the  endoskeleton,  the  former  having  reference  to  the  hard 
structures  found  superficial  to  the  soft  parts,  for  whose  pro- 
tection they  serve,  such  as  the  carapace  of  the  lobster,  and 
the  hard  scales  of  certain  fishes ;  while  the  latter  term 
signifies  the  cartilaginous  or  bony  structures  found  within 
the  bodies  of  most  vertebrate  animals.  Even  in  the  highest 
vertebrates,  certain  bones,  such  as  those  of  the  vault  of  the 
cranium,  are  usually  considered  by  morphologists  as  being 
the  representatives  of  part  of  the  exoskeleton  of  lower  types. 

The  skeleton,  using  the  word  in  its  ordinary  sense,  con- 
sists of  the  axial  skeleton  and  the  appendicular  skeleton,  or 
skeleton  of  the  limbs.  The  former,  including  the  head  and 
the  trunk,  is  common  to  all  vertebrates;  the  latter  is  not 
found  in  the  lowest  members  of  this  class  and  hence  is  to  be 
regarded  as  a  later  acquisition  in  the  evolution  of  the  skeleton. 

In  studying  the  development  of  the  skeleton,  as  in  con- 
sidering that  of  other  systems  and  organs,  clearer  conceptions 
of  the  growth  of  the  individual  may  be  obtained  by  com- 
paring it  with  the  evolution  of  the  type.  For  example,  the 
simplest  form  of  skeletal  apparatus  is  that  of  the  amphioxus. 
In  this  animal  the  only  representative  of  the  skeleton  is  the 

347 


348  TEXT-BOOK  OF  EMBRYOLOGY. 

notochord,  a  cylindrical  rod  composed  of  cellular  or  gelatinous 
tissue  in  which  neither  chondrification  nor  ossification  ever 
takes  place.  Such  an  animal  furnishes  an  example  of  the 
notochordal  stage  of  the  skeleton.  The  surrounding  of  the 
chorda  with  a  sheath  of  embryonal  connective  tissue,  by 
which  it  is  strengthened  and  thereby  better  fitted  to  serve  as 
the  body-axis,  furnishes  the  membranous  type  of  skeleton,  a 
stage  a  little  farther  advanced  than  the  preceding.  The  next 
higher  type  of  skeleton  is  the  cartilaginous  form.  In  this  case 
the  embryonal  connective  tissue  has  undergone  transformation 
into  cartilage,  at  which  point  development  is  arrested,  the 
stage  of  ossification  never  being  attained.  The  cartilaginous 
type  of  skeleton  is  illustrated  by  that  of  the  selachian  (sharks 
and  dog-fish). 

The  third  and  highest  type  of  skeleton  is  the  osseous.  This 
results  from  the  replacement  of  the  cartilaginous  tissue  by  bone. 
The  process  of  ossification  does  not,  however,  affect  every 
part  of  the  cartilaginous  skeleton,  there  being  some  portions 
of  the  latter  which  remain  permanently  unossified.  As  there 
are,  throughout  the  vertebrate  series  of  animals,  various  gra- 
dations in  the  degree  of  differentiation  of  the  skeleton,  so  in 
the  course  of  development  does  the  osseous  system  of  every 
higher  vertebrate  pass  through  these  stages  from  the  simplest 
condition,  that  of  the  notochordal  skeleton,  to  the  highest 
form  of  the  almost  completely  ossified  skeletal  apparatus. 

THE   AXIAL  SKELETON. 

The  axial  skeleton,  as  stated  above,  includes  the  bones  of 
the  trunk  and  those  of  the  head.  Logically  the  development 
of  the  former  will  first  claim  attention. 

The  Development  of  the  Trunk. 
The  Stage  of  the  Chorda. — The  formation  of  the 
chorda  dorsalis  or  notochord  is  the  earliest  indication  of  the 
axis  of  the  embryonic  body  and  it  will  be  recalled  that  it  is 
also  one  of  the  earliest  embryological  processes.  The  mode 
of  development  of  the  chorda  from  the  entodermal  epithelium 
has  been  described  at  p.  65.     The  chorda  serves  the  pur- 


THE  AXIAL  SKELETON.  349 

pose,  as  it  were,  of  an  axis  about  which  the  permanent  ver- 
tebral cokimn  and  a  part  of  the  skull  are,  at  a  much  later 
date,  built  up.  The  anterior  or  headward  termination  of  the 
chorda  corresponds  to  the  position  of  the  later  hypophysis,  or 
pituitary  Ijody,  and  thus  the  chorda  is  coextensive,  not  only 
with  the  vertebral  column,  but  also  with  a  portion  of  the 
cranium.  The  cells  of  the  chorda  enlarge  and  become  dis- 
tended with  fluid,  the  protoplasm  of  each  cell  being  reduced 
to  a  thin  layer.  The  peripheral  cells,  however,  constituting 
a  distinct  layer,  the  chordal  epithelium,  remain  small,  and  it 
is  by  their  proliferation  that  the  chorda  increases  in  size.  In 
the  amphioxus  the  chorda  is  the  only  "  skeleton  "  that  is  ever 
acquired,  and  in  this  animal  it  is  a  permanent  strudiLre.  In 
all  other  vertebrates  it  becomes  surrounded  by  embryonal 
connective  tissue,  mesenchyme,  which  latter  undergoes  chon- 
drification,  and  in  the  higher  types  ossification  also.  While 
in  some  of  the  lower  vertebrates,  as  in  certain  classes  of 
fishes,  the  chorda  persists  as  a  structure  of  more  or  less  im- 
portance, in  the  higher  members  of  the  series,  birds  and 
mammals,  it  retrogrades  as  the  processes  of  chondrification 
and  ossification  go  on,  until  finally  it  is  represented  only  by 
the  pulpy  centers  of  the  intervertehral  disks. 

The  Membranous  Stage. — The  notochordal  stage  of 
the  development  of  the  vertebral  column  is  succeeded  by  the 
membranous  stage.  The  transformation  is  effected  by  the 
appearance  of  an  ensheathing  mass  composed  of  embryonal 
connective-tissue  cells  which  surround  not  only  the  chorda 
but  also  the  neural  canal  or  fundament  of  the  nervous  system 
(Fig.  158,  sk).  The  source  of  this  embryonal  connective  tissue 
or  mesenchyme  bears  an  important  relation  to  the  primitive 
segments.  As  the  development  of  the  primitive  segments 
was  described  in  the  last  chapter,  and  also  in  Chapter  IV., 
it  will  suffice  to  remind  the  reader  that  each  primitive  seg- 
ment undergoes  differentiation  into  the  myotome  or  muscle- 
plate,  the  cutis-plate,  the  nephrotome,  and  the  sclerotome  (Fig. 
158),  the  sclerotome  occupying  the  mesial  surface  of  the  seg- 
ment and  lying  in  close  proximity  to  the  chorda. 

While  the  myotome  originates  from  the  flattened  or  meso- 


350 


TEXT-BOOK  OF  EMBRYOLOGY. 


thelial  cells  of  the  primitive  segment,  the  sclerotome  is  made 
up  of  cells  of  the  type  characteristic  of  young-growing  con- 
nective tissue — that  is,  of  the  mesenchymal  part  of  the  primi- 
tive segments  as  distinguished  from  their  mesothelium.  Owing 
to  the  rapid  multiplication  of  its  cells,  each  sclerotome  spreads 
out  headward  and  caudalward,  and  dorsad  and  ventrad,  sur- 
rounding both  the  chorda  and  the  neural  canal,  until  both  these 
structures  become  enclosed  in  a  common,  continuous  sheath  of 
embryonal  connective  tissue.  That  part  of  this  tissue  which 
surrounds  the  chorda  is  often  designated  the  skeletogenous 
sheath  of  the  chorda  and  also  the  membranous  primordial  ver- 
tebral column.  The  cells  of  the  sclerotomes  not  only  sur- 
round the  chorda  and  the  neural  canal,  but  they  also  spread 
out  laterally  into  the  intervals  between  the  muscle-segments 
to  constitute  the  ligamenta  intermuscularia  or  the  bands 
or  strips  of  connective  tissue  which  separate  adjacent  muscle- 


Micscle-sep-me7tts 


Intersegmental 

arteries 


l>" 


Skeletogenotis  sheath  of  chorda/  ^Chorda 


1st  spinal  nerve 

Ligame7ituni 

intermusai  larium 

2d  spinal  net-ve 

Liganientu77i 

intei-mtiscularitim 

jd  spinal  nerve 


Fig.  159.— Frontal  projection  from  a  series  of  sections  through  a  cow  embryo  of 
8.8  mm. (0.35  in.),     (From  Bonnet,  after  Froriep.) 


segments  from  each  other  (Fig.  159).  It  is  worthy  of  note 
that  while  this  skeletogenous  sheath  of  the  chorda  originates 
from  segmented  structures,  the  somites  or  primitive  segments, 
and  is  to  that  extent  related  to  the  segmentation  of  the  body, 
it  now  presents  no  trace  of  segmentation. 


THE  AXIAL  SKELETON. 


35i 


The  Cartilaginous  Stage. — This  stage  of  the  develop- 
ment of  the  spine  is  brought  about  by  the  metamorphosis  of 
parts  of  the  membranous  vertebral  column  into  the  car- 
tilaginous vertebrae.  Other  and  alternating  parts  of  the  same 
structure  furnish  the  intervertebral  disks  and  the  ligaments 
that  bind  together  the  individual  elements  of  the  spine.  The 
histological  changes  necessary  to  effect  the  transformation  of 
the  embryonal  connective  tissue  into  cartilage  are,  briefly, 
the  moving  apart  of  the  cells  and  the  modification  of  both 
the  cells  and  the  intercellular  substance,  the  latter  acquiring 
the  characteristic  qualities  of  the  matrix  of  cartilage. 

As  a  preliminary  step  to  the  formation  of  the  cartilaginous 
vertebrse,  the  ensheathing  membranous  tissue  exhibits,  at 
regular  intervals,  areas  of  condensation  of  its  connective- 
tissue  elements.  It  is  in  these  condensed  areas  that  the 
process  of  cartilage-formation  begins,  and  each  such  area, 
which  has  the  form  of  a  somewhat  obliquely  placed  bow  or 
half-arch,  corresponds  approximately  but  not  accurately  to  a 
future  vertebra.  This  half-arch  of  condensed  mesenchymal 
tissue  is  called  the  primitive  vertebral  bow  by  Froriep,  whose 


spinal  cord 


Center  of 
cho7idrifi- 
cation 


Spinal 
p'anp'lion 


Muscle- 
segment 

Chorda 
Primitive 

ve?-tebral 

bow 
Anlage    of 

rib 
Nerve 


Hypochordal  brace. 

Pig.  160. — Cross-section  through  the  anlage  of  the  third  cervical  vertebra  of  a  cow 

embryo  of  12  mm.  {\4.  in-)  (Bonnet). 


investigations  established  many  of  the  facts  known  concern- 
ing the  development  of  the  skeleton  (Fig.  160).  The  median 
part  of  the  bow  is  on  the  ventral  side  of  the  chorda  and  is 
known  as  the  hypochordal  brace.    The  extremities  of  the  bow 


352  TEXT-BOOK  OF  EMBRYOLOGY. 

abut  against  the  corresponding  muscle-segments,  each  extrem- 
ity becoming  bifurcated.  The  dorsal  limb  of  the  bifurcation 
spreads  over  the  dorsal  or  superficial  surface  of  the  primitive 
spinal  cord,  forming  the  membranous  forerunner  of  the 
neural  arch  of  the  vertebra ;  while  the  ventral  limb  advances 
ventrad,  foreshadowing  the  hemal  arch  or  costal  process  of 
the  vertebra,  or,  as  regards  the  thoracic  region  of  the  body, 
the  future  rib.^  The  lateral  parts  of  the  bow  become  the 
processes  of  the  vertebra,  but  the  median  part  of  each  bow, 
the  hypochordal  brace,  remains  unchondrified  in  mammals 
and  becomes  a  part  of  the  intervertebral  ligament,  except  in 
the  case  of  the  first  cervical  vertebra,  or  atlas,  the  anterior 
or  ventral  arch  of  which  it  furnishes.  The  membranous 
anlage  of  the  cartilaginous  body  of  the  vertebra  is  found  in 
a  special  condensation  of  the  ensheathing  tissue  of  the  chorda 
just  caudad  of  the  hypochordal  brace. 

For  each  vertebral  body  there  are  two  centers  of  chondri- 
fication,  one  on  each  side  of  the  chorda  within  the  mass  of 
tissue  referred  to  above  (Fig.  160).  The  formation  of  car- 
tilage begins  in  the  second  month.  The  two  centers  are  soon 
connected  with  each  other  by  a  third,  which  lies  on  the  ventral 
side  of  the  chorda,  the  three  forming  now  a  cartilaginous  half- 
cylinder  which  is  later  completed  by  the  development  of  car- 
tilage on  the  dorsal  side  of  the  chorda  (Fig.  161).  At  the  time 
when  the  chorda  is  completely  encased  in  cartilage  the  spinal 
cord  is  still  ensheathed  by  merely  membranous  tissue.  Before 
the  end  of  the  second  month  the  neural  arches  of  the  vertebrae 
are  indicated  by  small  isolated  masses  of  cartilage  which  de- 
velop in  the  connective  tissue  surrounding  the  spinal  cord, 
the  lateral  parts  of  the  membranous  vertebral  bows.  In  the 
eighth  week  these  fuse  with  the  bodies  and  appear  then  as 
projections  from  them.  By  the  end  of  the  third  month  the 
processes,  or  neural  arches,  have  grown  sufficiently  to  meet 

*  Morphologically,  each  vertebra  is  possessed  of  a  neural  arch,  for  the 
protection  of  the  spinal  cord ;  and  a  hemnl  arch  for  the  protection  of  the 
organs  of  circulation,  respiration,  and  digestion,  the  ribs  of  man  and  the 
higher  vertebrates  being  the  persistent  hemal  arches  in  the  region  of  the 
thorax. 


THE  AXIAL  SKELETON. 


353 


with  their  fellows  on  the  dorsal  side  of  the  spinal  cord,  and 
in  the  fourth  month  the  corresponding  arches  of  the  two 
sides  become  united,  thus  completing  the  cartilaginous  sheath 
of  the  cord. 

The  masses  of  connective  tissue  occupying  the  intervals 
between  the  vertebral  bodies  are  known  as  the  intervertebral 


Bolt)  of  occipital  vertebra   -'    '^^ 


First  spinal  nerve 


Dorsal  projections   of  the     '^    / 
bifurcated  primitive  ver- 
tebral bows 


^■t      Parachordal  cartilage 


•^  Beginning  of  cartilaginous 
body  of  occipital  vertebra 

\  J  Beginning  of  cartilaginous 
body  of  first  cervical  ver- 
tebra 

'^"^^^ —    Vertebral    bow    of    second 
^- i  cervical  vertebra 


Chorda 
Fig.  161.— Frontal  projection  from  a  series  of  sections  through  a  cow  emhryo 
of  17  mm.  (0.67  in.),  dorsal  view  (from  Bonnet,  after  Froriep):  Connective  tissue 
stippled ;  cartilage  white. 

ligaments.  Subsequently  they  become  the  intervertebral  disks. 
The  tissue  between  the  cartilaginous  arches  becomes  differ- 
entiated into  the  ligamenta  subflava. 

While  the  unseginented  skeletogenous  sheath  of  the 
chorda  is  gradually  differentiating  into  the  separate  elements 
of  the  cartilaginous  vertebral  column,  the  chorda  itself  begins- 
to  retrograde.  Within  the  bodies  of  the  vertebrae  its  devel- 
opment is  completely  arrested,  while  those  portions  of  it  con- 
tained within  the  intervertebral  disks  continue  to  grow.  The 
chorda  at  this  stage  consequently  shows  alternating  enlarge- 
ments and  constrictions.  In  certain  fishes  it  persists  as  a 
structure  of  more  or  less  importance.  In  vertebrates  above 
cartilaginous  fishes,  all  traces  of  the   parts  of  the  chorda 

23 


354  TEXT-BOOK  OF  EMBRYOLOOY. 

within  the  vertebral  bodies  are  lost  as  soon  as  ossification 
occurs,  while  in  the  intervertebral  disks  parts  of  it  persist 
as  the  soft  pulpy  cores  of  the  latter. 

The  cartilaginous  trunk  is  completed  by  the  chondrification 
of  the  ligamenta  intermuscularia  to  form  the  cartilaginous 
thorax. 

The  Osseous  Stage. — The  process  of  ossification  begins 
in  certain  parts  of  the  trunk  at  the  end  of  the  second  month, 
before  the  work  of  chondrification  is  entirely  completed.  As 
the  histological  details  of  bone-formation  are  to  be  found  in 
th6  text-books  of  histology,  it  will  not  be  necessary  to  enter 
into  the  subject  here.  The  places  in  any  individual  cartilage 
where  ossification  begins  are  called  the  centers  of  ossification. 
The  process  is  one  of  substitution,  the  cartilage  becoming 
broken  down  and  absorbed  as  the  formation  of  bone  goes  on. 

The  ossification  of  each  vertebra  is  begun  at  three  cen- 
ters, one  in  the  body  and  one  in  each  arch.  The  centers 
for  the  arches  appear  in  the  seventh  week.  The  centers 
for  the  bodies  appear  a  little  later  and  are  found  first  in  the 
dorsal  vertebrae,  appearing  successively  later  in  the  verte- 
brae farther  up  and  farther  down.  The  ossified  arches  unite 
with  each  other  during  the  first  year  of  life,  but  their  union 
with  the  body  of  the  vertebra  takes  place  between  the  third 
and  eighth  years.  At  a  much  later  period  five  accessory  cen- 
ters of  ossification  are  added  to  each  vertebra.  Two  of  these 
belong  to  the  body  and  give  rise  to  two  annular  plates  of 
bone,  the  epiphyses,  one  for  the  upper  or  cephalic  surface  and 
one  for  the  opposite  or  caudal  surface.  The  remaining  three 
centers  belong  respectively  to  the  spinous  process  and  the  two 
transverse  processes.  The  epiphyses  do  not  acquire  osseous 
union  with  the  vertebra  proper  until  about  the  twenty-fifth 
year. 

The  so-called  transverse  process  of  a  cervical  vertebra,  en- 
closing a  foramen,  and  consisting  of  an  anterior  and  a  posterior 
part,  includes  more  than  the  transverse  process  proper,  since 
its  anterior  or  ventral  portion  is  the  rudiment  of  a  cervical  rib. 
During  the  time  of  the  fusion  of  this  rudimentary  rib  with 
the  transverse   process,  the  vertebral   artery,  which   passes 


THE  AXIAL  SKELETON.  -        •       355 

between  them,  is  surrounded  by  the  two  processes,  and  thus 
the  adult  cervical  transverse  processes  differ  from  those  of 
the  other  vertebrae  in  the  possession  of  a  foramen.' 

The  atlas  and  the  axis,  being  strikingly  modified  cervical 
vertebrae,  require  special  mention.  The  atlas  contains  less 
and  the  axis  more  than  an  ordinary  vertebra,  since  that  which 
corresponds  to  the  body  of  the  atlas  never  unites  with  it  but 
fuses  with  the  body  of  the  axis  to  constitute  its  odontoid 
process. 

The  atlas  presents  two  centers  of  ossification  for  its  neural 
arches — the  so-called  posterior  arch — just  as  other  vertebrse 
do.  Unlike  other  vertebrse,  these  centers  do  not  unite  with 
the  body  but  become  joined  to  each  other  on  the  ventral  side 
of  the  position  of  the  chorda  by  a  piece  of  cartilage  which 
results  from  the  chondrification  of  the  hypochordal  brace, 
referred  to  on  page  352.  This  forms  the  cartilaginous  ven- 
tral or  anterior  arch  of  the  atlas,  which,  in  the  first  year  of 
life,  develops  a  center  of  ossification.  The  arch  acquires 
bony  union  with  the  lateral  parts  between  the  fifth  and 
sixth  years. 

The  axis  or  epistropheus  develops  from  the  usual  centers  of 
ossification  and  from  an  additional  one  for  its  odontoid  proc- 
ess. Bony  union  of  the  odontoid  process  with  the  proper 
body  of  the  axis  occurs  in  the  seventh  year.  The  odontoid 
process,  in  common  with  every  other  vertebral  body,  is 
traversed  in  the  cartilaginous  stage  by  the  notochord. 

The  transverse  processes  of  the  lumbar  vertebrse,  like  those 
in  the  cervical  region,  include  not  only  the  transverse  proc- 
ess proper  but  also  the  rudiment  of  a  rib. 

The  sacral  vertebrse  each  present  the  usual  ossific  centers. 
Inasmuch  as  they  become  articulated  firmly  with  the  pelvic 
bones  and  undergo  fusion  to  form  a  single  adult  bone,  the 
sacrum,  their  form  is  much  modified  during  the  course  of 
development.  The  transverse  processes  of  each  side  coalesce  to 
form  the  lateral  mass  of  the  sacrum.  Each  transverse  process 

*  The  point  is  made  by  some  authorities,  as  Minot,  that  the  bone  does 
not  grow  around  the  artery,  but  that  the  artery  grows  through  the  ossifying 
tissue. 


356       •  TEXT-BOOK  OF  EMBRYOLOGY. 

consists,  as  in  the  cervical  and  the  lumbar  vertebrae,  of  the 
transverse  process  proper  and  a  rudimentary  rib,  the  center  of 
ossification  for  the  latter  being  quite  distinct  during  early 
stages  of  development.  The  intervertebral  disks  of  the  sacral 
vertebrae  begin  to  ossify  in  the  eighteenth  year,  the  process 
being  completed  in  the  twenty-fifth  year. 

The  coccygeal  vertebrae  are  quite  rudimentary.  Each  one 
is  ossifi'ed  from  a  single  piece  of  cartilage,  and  usually  from 
but  a  single  center  of  ossification.  Occasionally  the  first 
piece  of  the  coccyx  develops  from  two  ossific  centers,  the  proc- 
ess beginning  at  birth.  Ossification  begins  in  the  second 
vertebra  between  the  fifth  and  the  tenth  years  ;  in  the  third, 
shortly  before  puberty  ;  in  the  fourth,  soon  after  puberty. 
The  lower  three  pieces  fuse  into  one  before  middle  life,  and 
this  unites  with  the  first,  and  the  latter  with  the  sacrum,  at 
variable  periods  thereafter. 

The  Development  of  the  Ribs  and  Sternum. — 
Reference  has  been  made  in  the  preceding  pages  to  the  liga- 
menta  intermuscularia  as  strips  or  bands  of  embryonal  con- 
nective tissue  lying  between  adjacent  muscle  segments,  which 
have  originated,  in  common  with  the  sheath  of  the  chorda, 
from  the  cells  of  the  sclerotomes.  The  ligamenta  intermus- 
cularia become  invaded  by  the  costal  processes  of  the  primi- 
tive vertebral  bows,  the  costal  process,  which  is  the  ventral 
division  of  the  tip  of  the  bow,  growing  ventrad  and  pene- 
trating the  substance  of  the  ligament  to  constitute  a  curved 
rod  of  connective  tissue,  the  forerunner  of  the  future  rib. 
Thus  there  are  formed  connective-tissue  representatives  of 
the  ribs,  each  of  Avhich  is  embedded  in  the  looser  connective 
tissue  of  the  corresponding  intermuscular  ligament.  It  is 
by  the  development  of  cartilage  within  these  curved  rods  of 
condensed  mesenchyme,  the  membranous  ribs,  that  the  cartil- 
aginous ribs  are  produced.  The  process  of  chondrification 
commences  in  the  second  month,  but  does  not  involve  the 
proximal  ends  of  the  ribs,  the  tissue  here  becoming  liga- 
mentous and  serving  to  bind  together  the  ribs  and  the  verte- 
bne.  Ribs  are  formed  throughout  the  entire  extent  of  the 
vertebral  column,  except  in  the  coccygeal  region,  but  while  in 


THE  AXIAL  SKELETON.  .        357 

the  lower  vertebrates  the  entire  series  goes  on  to  mature  de- 
velopment, in  mammals,  including  man,  their  growth  is 
arrested  in  the  cervical,  lumbar,  and  sacral  regions.  In  the 
case  of  man  and  mammals  only  the  thoracic  ribs  persist  and 
become  adult  structures. 

As  the  distal  (ventral)  extremities  of  the  ribs  advance 
toward  the  ventral  median  line,  the  tips  of  the  first  five,  six, 
or  seven  each  exhibit  an  enlargement.  These  broadeaed  ends 
soon  coalesce,  thus  forming  on  either  side  of  the  median  line 
a  continuous  strip  of  cartilage,  the  anlages  of  the  sternum. 
The  other  ribs  remain  free  at  their  ends.  The  sternum  is 
therefore  produced  from  two  lateral  halves,  a  circumstance 
that  explains  some  of  its  anomalies,  as  for  example,  cleft 
sternum,  which  is  a  condition  due  to  arrested  development 
or  deficiency  of  union. 

The  ossification  of  the  ribs  begins  in  the  second  month  of 
fetal  life  and  from  a  single  center  for  each.  The  process  does 
not  involve  the  entire  rib,  a  portion  near  the  distal  extremity 
remaining  cartilaginous  and  becoming  the  adult  costal  carti- 
lage. Accessory  centers  of  ossification  for  the  head  and 
tubercle  appear  between  the  eighth  and  fourteenth  years 
of  life. 

The  ossification  of  the  sternum  proceeds  from  numerous 
centers.  There  is  one  for  the  manubrium  and  from  six  to 
twelve  for  the  gladiolus.  The  ensiform  acquires  a  center  of 
ossification  in  the  early  years  of  life,  but  for  the  most  part 
remains  cartilaginous. 

Although,  as  stated  above,  the  ribs  of  adult  human  anat- 
omy are  limited  to  the  thoracic  region,  their  rudimentary 
representatives  are  found  throughout  the  other  regions  of 
the  vertebral  column.  In  the  cervical,  lumbar,  and  sacral 
regions  each  rudimentary  rib  becomes  blended  with  the 
transverse  process  of  the  corresponding  vertebra  to  form  the 
transverse  process  of  human  anatomy.  It  is  from  the  per- 
sistence of  the  seventh  rudimentary  cervical  rib  and  its  fail- 
ure to  fuse  with  the  corresponding  transverse  process  that 
the  anomaly  of  a  free  cervical  rib  results ;  while  the  presence 
of  a  thirteenth  or  lumbar  rib,  as  occasionally  met  with,  is  due 


358 


TEXT-BOOK  OF  EMBRYOLOGY. 


to  the  unusual  development  of  the  first  lumbar  rudimentary 
rib. 

The  Development  of  the  Head  Skeleton. 

Just  as  the  skeleton  of  the  trunk  consists  of  a  dorsally 
situated  bony  case  for  the  protection  of  the  spinal  cord  and  a 
series  of  ventral  or  hemal  arches  for  the  protection  of  the 
organs  of  circulation  and  respiration ;  so  does  the  head 
skeleton  comprise  a  bony  case  for  the  accommodation  of  the 
brain  with  smaller  accessory  osseous  compartments  for  the 
organs  of  special  sense,  as  the  orbits  and  the  nasal  chambers; 
and  also  a  ventrally  situated  apparatus  which  constitutes 
both  a  receptacle  for  the  oral  and  the  pharyngeal  parts  of 
the  digestive  system  and  a  mechanism  for  the  mastication  of 


Fig.  162.— Median  sagittal  section  through  the  head  of  a  chick  incubated  four 
and  a  half  days  (after  Mihalkovics) :  SH,  parietal  (mid-brain)  elevation ;  sv,  lateral 
ventricle  of  the  brain;  v»,  third  ventricle;  iA,  fourth  ventricle;  Sw,  aqueductus 
Sylvii;  gk,  cerebral  vesicle;  zh,  betv?een-brain  (thalamencephalon) ;  m/i,]  mid- 
brain ;  kh,  cerebellum ;  zf,  pineal  process ;  hp,  hypophyseal  (or  Rathke's)  pocket ; 
ch,  chorda ;  6a,  basilar  artery. 

food.  The  former  part,  the  cranial  capsule,  or  brain-case,  is 
developed  to  a  great  extent  from  the  connective  tissue  sur- 
rounding the  head-end  of  the  chorda,  its  origin  thus  being 
similar  to  that  of  the  spinal  column.  On  the  other  hand, 
the  ventral  parts,  as  the  jaws  and  the  hyoid  bone  and  related 
structures,  constituting  the  so-called  visceral  skeleton,  develop 
from  the  mesodermic  tissue  of  the  visceral  arches.     As  in  the 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   359 

case  of  the  trunk  skeleton,  the  cranium  is  first  outlined  in 
membranous  tissue  resulting  from  the  differentiation  of  the 
embryonal  connective  tissue  which  ensheaths  the  head-end 
of  the  chorda,  and  also  of  the  connective  tissue  of  the  visceral 
arches,  this  differentiation  producing  the  membranous  primor- 
dial cranium.  The  metamorphosis  of  the  membranous  cranium 
into  cartilage  brings  about  the  cartilaginous  stage  of  the 
cranium,  while  the  replacement  of  the  cartilage  by  bone  is 
the  final  step  in  the  process. 

Bones  that  develop  from  centers  of  ossification  in  pre- 
viously formed  masses  of  cartilage  are  styled  primordial 
bones,  while  those  that  are  produced  independently  of  car- 
tilage, either  in  the  skin  covering  the  membranous  cranium, 
or  in  the  mucous  membrane  lining  indentations  in  its  walls, 
are  known  as  covering  or  dermal  bones.  The  development 
of  bone  is  therefore  said  to  be  either  endochondral  or  mem- 
branous. For  the  most  part,  the  bones  of  the  base  of  the 
skull  are  of  endochondral  formation,  while  those  of  the  vault 
are  developed  in  membrane.  The  membranous  or  dermal 
bones  are  similar  in  point  of  origin  to  the  exoskeleton — 
placoid  and  ganoid  scales — of  certain  fishes. 

The  Membranous  Cranium. — The  membranous  brain- 
case  is  differentiated  from  the  young  connective  tissue  which 
ensheaths  the  anterior  or  head-end  of  the  chorda.  As  pre- 
viously stated,  the  anterior  end  of  the  chorda  is  at  a  point 
ventrad  to  the  mid-brain  vesicle,  in  the  angle  formed  by  the 
latter  with  the  fore-brain,  at  a  position  corresponding  with 
that  of  the  pituitary  body  (Fig.  162).  The  skeletogenous 
sheath  of  the  chorda,  in  this  situation  as  elsewhere,  results 
from  the  multiplication  of  the  cells  of  the  sclerotomes,  since 
this  region  of  the  body  undergoes  segmentation  in  common 
with  the  trunk.  The  number  of  head-segments  is  uncertain. 
According  to  recent  investigations  upon  shark  embryos,  there 
are  at  least  nine  primitive  segments  formed  in  the  head- 
region. 

The  skeletogenous  sheath  of  the  chorda  spreads  out  dorsad 
to  cover  the  brain-vesicles.  From  the  terminal  point  of  the 
chorda,  beneath  the  inter-brain,  the  sheath  advances   ante- 


360  TEXT-BOOK  OF  EMBRYOLOGY. 

riorly  to  invest  the  fore-brain,  which  latter  at  this  stage  is 
bent  over  ventrad.  From  the  part  investing  the  fore-brain, 
a  protuberant  mass,  the  nasofrontal  process,  extends  toward 
the  primitive  mouth-cavity,  constituting  the  anterior  or  upper 
boundary  of  the  latter.  Meanwhile  the  mesenchymatic  tis- 
sue of  the  visceral  arches — that  is,  that  part  of  the  meso- 
dermic  tissue  of  these  structures  which  does  not  form 
muscular  tissue — is  undergoing  similar  transformation  into 
membranous  tissue.  The  first  visceral  arch  divides  into  an 
anterior  or  upper  part,  the  maxillary  process,  and  a  posterior 
or  lower  mass,  the  mandibular  arch,  these  being  the  mem- 
branous jaw  arches.  The  four  jaw  arches,  with  the  naso- 
frontal process,  form  the  boundaries  of  the  primitive  mouth- 
cavity,  the  mandibular  ai'ches  of  the  two  sides  having  iniited 
in  the  median  line  to  form  its  lower  border,  and  the  maxillary 
arches  having  fused  with  the  lateral  nasal  and  the  nasofrontal 
processes  to  constitute  its  upper  boundary. 

The  membranous  primordial  cranium,  then,  consists  of  a 
complete  connective-tissue  investment  for  the  brain-vesicles, 
of  the  membranous  jaw  arches,  and  of  the  hyoid  and  the 
branchial  arches,  and  presents  in  its  walls  the  indications  of 
the  cavities  for  special-sense  organs  in  the  shape  of  the  sur- 
face invaginations  wdiich  constitute  respectively  the  otic  ves- 
icle, the  lens-vesicle,  and  the  nasal  pits. 

The  Cartilaginous  Cranium. — By  the  further  differ- 
entiation of  the  membranous  cranium  the  cartilaginous  stage 
is  attained.  The  development  of  cartilage  begins  in  the 
second  month.  While  the  membranous  cranium  furnishes  a 
complete  capsule  for  the  brain,  the  cartilaginous  brain-case  is 
deficient,  since  the  process  of  chondrification  does  not  affect 
the  regions  of  the  future  parietal  and  frontal  bones.  This  is 
true  at  least  of  man  and  the  higher  vertebrates.  In  those 
cases  where  the  skeleton  remains  permanently  cartilaginous, 
as  in  selachians  (sharks,  dog-fish,  etc.),  the  entire  brain-case 
participates  in  the  chondrifying  process.  As  the  skull  ex- 
tends very  much  farther  forward  than  the  end  of  the  chorda 
— which  latter  terminates  at  the  position  of  the  future  sella 
turcica — the  regions  of  the   primitive  skull  are  designated 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   361 

respectively  chordal  and  prechordal  (K5lliker),  or  vertebral 
and  evertebral  (Gegenbauer),  according  as  they  fall  behind  or 
in  front  of  the  end  of  the  chorda. 

The  formation  of  cartilage  begins  in  the  region  correspond- 
ing to  the  base  of  the  future  skull.  On  each  side  of  the  end 
of  the  chorda  a  mass  or  bar  of  cartilage  is  formed,  extending 
forward  and  backward,  this  pair  of  parallel  bars  being  desig- 
nated the  parachordal  cartilages  (Fig.  1 63, 1).    Farther  forward, 


Fig.  163.— First  fundament  of  the  cartilaginous  primordial  cranium  (from 
Wiedersheim) :  1.  First  Stage:  C,  chorda;  PB,  parachordal  cartilage ;  Tr,  Rathke's 
trabeculse  cranii;  PR,  passage  for  the  hypophysis  ;  N,  A,  0,  nasal  pit,  optic  vesi- 
cle, otocyst.  2.  Second  Stage:  C,  chorda;  B,  basilar  plate;  Tr,  trabeculse  cranii, 
which  have  become  united  in  front  to  constitute  the  nasal  septum  (S)  and  the  eth- 
moid plate ;  Ct,  AF,  processes  of  the  ethmoid  plate  enclosing  the  nasal  organ  ;  01, 
foramina  olfactoria  for  the  passage  of  the  olfactory  nerves  ;  PF,  postorbital  proc- 
ess ;  NK,  nasal  pit;  A,  0,  optic  and  labyrinthine  vesicles. 

in  the  prechordal  region,  another  pair  of  cartilaginous  masses 
is  produced,  known  as  the  trabeculae  cranii.  The  latter  are 
not  straight  bars,  but  have  somewhat  the  form  of  a  pair  of 
calipers.  In  a  short  time  the  cranial  trabeculae  unite  with 
each  other,  but  not  throughout  their  entire  extent,  an  aperture 
being  left  at  the  position  of  the  pituitary  body.  It  is  through 
this  aperture  that  the  oropharyngeal  diverticulum,  which 
forms  the  anterior  lobe  of  the  pituitary  body,  projects  to 
come  into  relation  with  the  diverticulum  from  the  inter-brain, 
which  produces  the  posterior  lobe.     At  a  later  period  ossifi- 


362  TEXT-BOOK  OF  EMBRYOLOGY. 

cation  occurs  here,  as  elsewhere  in  the  base  of  the  skull,  thus 
completely  isolating  the  pituitary  body  from  the  wall  of  the 
pharynx.  The  parachordal  cartilages  also  fuse  with  each 
other  and  with  the  cranial  trabeculse,  the  four  pieces  now 
forming  one  mass.  The  process  of  chondrification  extends  to 
other  parts  of  the  membranous  cranium  so  as  to  produce  a 
cartilaginous  brain-case,  just  as,  in  the  case  of  the  vertebral 
column,  the  dorsal  extension  of  cartilage-formation  gives  rise 
to  a  case  or  canal  for  the  spinal  cord.  As  before  stated,  how- 
ever, the  chondrifying  process  does  not  affect  the  entire 
membranous  cranium  in  the  higher  vertebrates,  chondrifica- 
tion occurring  around  the  position  of  the  foramen  magnum 
and  in  the  lateral  walls  of  the  cranial  capsule,  while  parts  of 
the  vault  remain  membranous.  The  anterior  extremities  of 
the  united  cranial  trabeculse  become  so  modified  in  form  as  to 
constitute  the  plate  of  the  ethmoid  and  the  nasal  capsule  for 
the  lodgement  of  the  olfactory  epithelium.  In  each  lateral 
region  the  cartilaginous  ear  capsule  is  differentiated. 

Meanwhile  the  cartilaginous  visceral  skeleton  is  developing 
from  the  membranous  structures  of  the  visceral  arches.  As 
in  the  case  of  the  brain-capsule,  the  chondrifying  process  does 
not  involve  all  parts  of  the  membranous  visceral  skeleton, 
parts  of  the  latter  being  replaced  later  by  dermal  or  covering 
bones — that  is,  bones  that  develop  in  membrane  without 
having  been  previously  mapped  out  in  cartilage. 

In  the  first  visceral  arch,  the  formation  of  cartilage  occurs 
only  in  the  mandibular  portion,  the  maxillary  process  con- 
tinuing membranous.  The  cartilage  of  the  mandibular  arch 
appears  in  the  form  of  a  curved  bar  running  ventrodorsally. 
This  bar  divides  into  a  smaller  proximal  or  dorsal  piece,  the 
palatoquadratura  of  comparative  anatomy,  and  a  longer  distal 
or  ventral  segment,  Meckel's  cartilage.  The  palato-quadratum 
subsequently  divides  into  two  parts,  the  cartilaginous  anlages 
respectively  of  the  palato-pterygoid  plate  and  the  incus. 
Meckel's  cartilage  likewise  undergoes  division,  there  being 
separated  from  the  chief  mass  a  small  proximal  segment 
called  the  articulare,  Avhich  is  the  forerunner  of  the  future 
malleus.     Thus  the  cartilaginous  bar  of  the  mandibular  arch 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   363 

has  to  do  with  the  formation  of  certain  of  the  ossicles  of  the 
middle  ear  as  well  as,  to  a  limited  extent,  with  the  develop- 
ment of  the  mandible. 

In  the  second  visceral  or  anterior  hyoid  arch,  chondrifica- 
tion  also  occurs,  but  not  throughout  its  entire  extent.  A  bar 
of  cartilage,  the  hyoid  bar  or  Eeichert's  cartilage,  is  produced 
in  this  arch  and  undergoes  division  into  three  segments,  of 
which  the  proximal  or  dorsal  is  the  forerunner  of  the  future 
stapes  of  the  middle  ear,  while  the  other  two  pieces  represent 
respectively  the  styloid  process  and  the  lesser  horn  of  the 
hyoid  bone.  The  tissue  intervening  between  the  position  of 
the  styloid  process  and  the  lesser  hyoid  cornu  does  not  chon- 
drify  in  man  but  remains  membranous  and  becomes  the  stylo- 
hyoid ligament  (see  Fig.  169). 

In  the  third  visceral  arch,  or  the  posterior  hyoid  arch,  a  rod 
of  cartilage  develops  which  represents  the  greater  cornu  of 
the  future  hyoid  bone.  Ventral  to  this,  there  is  formed  a 
median  unpaired  piece  of  cartilage,  the  copula,  belonging  to 
the  arches  of  the  two  sides,  which  later  develops  into  the 
body  of  the  os  hyoides. 

To  summarize,  the  head  skeleton  in  the  cartilaginous  stage 
of  development  presents  an  imperfect  cartilaginous  brain-case, 
capsules  for  the  organs  of  smell,  sight,  and  hearing,  and  a 
cartilaginous  visceral  skeleton,  the  several  parts  of  which  map 
out  the  lower  jaw,  the  hyoid  bone,  the  styloid  process,  and 
the  ossicles  of  the  middle  ear. 

The  Osseous  Stage. — The  bony  condition  of  the  head 
skeleton  is  brought  about  in  part  by  the  development  of  bone 
from  centers  of  ossification  in  the  cartilages  described  above,  and 
in  part  by  the  growth  of  covering  or  dermal  bones  in  the  integu- 
ment covering  those  areas  which  are  deficient  in  cartilage ;  in 
other  words,  by  both  endochondral  and  membranous  ossifica- 
tion. It  may  be  stated  in  general  terms  that  the  bones  of  the 
base  and  of  the  sidesof  the  skull,  including  the  auditory  ossicles, 
the  ethmoid,  and  the  inferior  turbinated  bone,  are  produced  by 
ossification  in  cartilage  and  are  hence  called  'primordial  bones; 
and  that  the  bones  of  the  vault  of  the  cranium,  and  for  the 
most  part  of  the  face,  result  from  the  membranous  method  of 


364 


TEXT-BOOK  OF  EMBRYOLOGY. 


Fig.  164.— Tabular  part  of  oc- 
cipital bone  of  about  fiftli  fetal 
month,  inner  surface :  ip,  inter- 
parietal, which  is  ossified  in  mem- 
brane; so,  supra-occipital,  ossified 
in  cartilage. 


ossification,  and  are  therefore  styled  dermal  or  covering  bones. 
Some  of  the  individual  bones,  however,  are  partly  of  car- 
tilaginous and  partly  of  mem- 
branous origin,  the  several  por- 
tions remaining  permanently  dis- 
tinct in  certain  lower  vertebrates, 
but  in  man  uniting  so  intimately 
with  each  other  as  to  present  no 
trace  of  their  previously  separate 
condition. 

The  occipital  hone  consists  of 
two  genetically  distinct  parts, 
the  superior  or  interparietal  por- 
tion, which  is  a  dermal  bone, 
and  the  occipital  bone  proper, 
which  is  of  cartilaginous  origin.  The  ossification  of  the  latter 
occurs  from  four  centers,  one  on  each  side  of  the  foramen 
magnum  for  the  condylar  portions,  one  in  front  of  the  foramen 
for  the  basilar  process,  and  one  posterior  to  that  aperture  for 
all  the  tabular  portion  of  the  bone  not  belonging  to  the  inter- 
parietal segment.  Ossification  begins  in  these  centers  early 
in  the  third  fetal  month  and  proceeds  at  such  rate  that  at  the 
time  of  birth  the  bone  consists  of  four  bony  parts  which  are 
separated  from  each  other  merely  by  thin  layers  of  cartilage. 
Since  in  some  animals  these  parts  remain  separate  throughout 
life,  they  are  designated  by  morjihologists,  respectively,  the  ex- 
occipitals,  the  basi-occipital,  and  the  supra-occipital  (Fig.  165). 
The  supra-occipital  is  augmented  by  the  union  with  it  of  the  in- 
terparietal portion,  a  covering  or  dermal  bone  that  ossifies  from 
two  centers,  and  that  begins  to  fuse  with  the  supra-occipital 
near  the  end  of  the  third  month  of  fetal  life.  Consisting  at 
birth  of  four  distinct  parts,  separated  by  cartilage,  the  occip- 
ital becomes  a  single  bone  by  the  end  of  the  third  or  fourth 
year  by  the  bony  union  of  the  separate  segments.' 

The  temporal  bone  is  made  up  of  three  genetically  distinct 

^  In  some  case.s  the  union  of  the  interparietal  with  the  supra-occipital  is 
incomplete,  the  adult  bone  then  presenting  two  transverse  fissures  which 
pass,  one  from  each  lateral  angle,  toward  the  median  line. 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.  365 

parts,  the  squamosal  or  sc[uamozygoniatic,  the  petrosal  or  petro- 
mastoid  or  periotic,  and  the  tympanic.  At  the  time  of  birth 
these  three  elements  of  the  bone  are  still  separate  from  each 
other,  the  tympanic  being  an  incomplete  ring,  and  the  petro- 


FiG.  165. — Occipital  bone  at  birth,  external  surface :  ip,  interparietal ;  so,  supra- 
occipital  ;  eo,  exoccipitals ;  bo,  basi-occipital. 

mastoid  being  still  without  a  mastoid  process.  The  petro- 
mastoid  is  the  only  part  of  the  temporal  bone  that  is  outlined 
in  cartilage,  the  squamozygomatic  and  the  tympanic  being 
represented  in  the  cartilaginous  stage  of  the  cranium  by 
membranous  tissue. 

The  squamozygomatic  (Fig.  166)  is  ossified  in  previously 


366 


TEXT-BOOK  OF  EMBRYOLOGY. 


Fig.  166.  —  Squamozygo- 
matic  (sg)  and  tympanic  (<), 
of  temporal  bone  at  birth. 


formed  membrane  from  a  single  center  of  ossification,  which 
appears  in  the  lower  part  of  this  segment  at  about  the  seventh 
week.  The  process  of  bone-formation  extends  in  all  direc- 
tions from  this  center,  but  especially 
upward  into  the  squamosa  and  out- 
ward and  forward  into  the  zygoma. 
The  periotic  or  petromastoid  results 
from  the  ossification  of  the  cartilagi- 
nous ear-capsule,  which  latter  consti- 
tutes a  part  of  the  cartilaginous  por- 
tion of  the  early  cranium.  It  should 
be  remembered  that  the  essential  part 
of  the  organ  of  hearing,  the  internal 
ear,  is  differentiated  from  a  small 
pouch  of  epithelium,  the  otic  vesicle, 
which  is  produced  by  an  infolding  or 
invagination  of  the  surface  ectoderm,  and  that  it  is  the  car- 
tilaginous tissue  enclosing  the  otic  vesicle  and  its  outgrowths, 
the  semicircular  canals  and  the  cochlea,  that  constitutes  the 
cartilaginous  ear-capsule. 

The  ossification  of  the  periotic  is  usually  described  as  pro- 
ceeding from  three  centers.  The  first  of  these,  the  opisthotic, 
makes  its  appearance  in  the  latter  part  of  the  fifth  month  on 
the  outer  wall  of  the  capsule,  at  a  point  corresponding  to  the 
position  of  the  promontory,  whence  the  formation  of  bone 
spreads  in  such  manner  as  to  produce  that  part  of  the  petrosa 
which  is  below  the  internal  auditory  canal.  A  second  center, 
the  pro-otic,  appears  a  little  later  over  the  superior  semi- 
circular canal  and  gives  rise  to  that  part  of  the  petrosa  above 
the  internal  auditory  meatus,  and  also  to  the  inner  and  upper 
part  of  the  mastoidea.  The  third  nucleus,  the  epiotic,  arises 
in  the  neighborhood  of  the  posterior  semicircular  canal. 
Ossification  proceeds  rapidly,  the  three  parts  speedily  uniting 
to  form  one  bone,  the  periotic  or  petromastoid.  The  petrous 
portion  of  the  periotic  is  the  more  important  and  the  more 
constant.  The  mastoid  is  of  variable  size  in  different  ani- 
mals, and  in  the  human  species,  at  birth,  it  is  flat  and  devoid 
of  the  mastoid  process  which  is  so  conspicuous  in  the  mature 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   367 

condition  of  the  skull.  The  mastoid  process  develops  during 
the  first  two  years  of  life,  but  its  air-cells  do  not  appear  until 
near  the  age  of  puberty. 

The  pars  tympanicus,  or  the  tympanic  (Fig.  166),  which  con- 
stitutes the  bony  part  of  the  wall  of  the  external  auditory  me- 
atus, is  ossified  in  membrane  from  a  single  center  of  ossification. 
This  center  appears  in  the  third  fetal  month  in  the  lower  part 
of  the  membranous  wall  of  the  external  canal,  from  which 
point  the  process  of  bone-formation  extends  upward  on  either 
side  so  as  to  form  an  incomplete  bony  ring,  open  above. 
This  tympanic  ring  is  situated  external  to  both  the  ear  cap- 
sule and  the  ossicles  of  the  middle  ear  and  gives  attachment 
to  the  periphery  of  the  tympanic  membrane.  The  further 
growth  of  the  tympanic  ring  being  in  the  outward  direction, 
it  becomes  a  curved  plate  or  imperfect  cylinder  of  bone 
which  constitutes  the  bony  wall  of  the  external  auditory 
canal.  At  birth,  the  pars  tympanicus  still  has  the  form  of 
the  incomplete  ring,  its  further  development  taking  place 
during  the  first  few  years  of  life.  The  extremities  of  the 
ring  unite  with  the  squamozygomatic  before  birth.  The 
tympanic  unites  also  with  the  petrosa  except  in  a  region 
adjacent  to  the  proximal  end  of  Meckel's  cartilage,  where 
an  aperture  is  left  which  is  the  petrotympanic  or  Glaserian 
fissure.  Since  upon  the  part  of  Meckel's  cartilage  which  is 
thus  enclosed  by  the  union  of  the  two  bones  is  formed  the 
long  process  of  the  malleus,  the  presence  of  this  process  in 
the  Glaserian  fissure  is  accounted  for. 

The  styloid  process  of  the  temporal  bone  belongs  to  the 
visceral-arch  skeleton.  It  ossifies  in  two  parts  in  small 
masses  of  cartilage  that  belong  to  the  anterior  hyoid  arch. 
One,  the  tympanohyal,  gives  rise  to  the  base  of  the  process 
(Fig.  170);  it  begins  to  ossify  before  birth  and  soon  unites 
with  the  temporal.  The  other  segment,  the  stylohyal,  under- 
goes ossification  later  and  joins  with  the  tympanohyal  only 
after  adult  age  is  reached.  Sometimes  it  remains  separate 
throughout  life. 

The  sphenoid  bone  is  for  the  most  part  ossified  in  cartilage. 
The  body  of  the  bone  is  represented  in  the  fetus  by  two 


368 


TEXT-BOOK  OF  EMBRYOLOGY. 


separate  parts,  the  posterior  body  or  basisphenoid  (Fig.  167, 
bs),  which  includes  all  that  part  of  the  body  of  the  mature 
bone  which  is  posterior  to  the  olivary  eminence  and  to 
which  belong  the  greater  wings;  and  an  anterior  body  or 
presphenoid  {ps),  situated  in  front  of  the  olivary  eminence, 
to  which  belong  the  lesser  wings.  The  ossification  of 
the  basisphenoid  proceeds  from  two  centers  placed  side  by 


Fig.  167.— Sphenoid  bone,  fifth  or  sixth  fetal  month;  seen  from  above:  ps,  pre- 
sphenoid  or  anterior  body,  with  lesser  wings ;  as,  greater  wings ;  bs,  basisphenoid 
or  posterior  body. 


side,  which  appear  in  the  eighth  week.  Two  months  later 
two  secondary  centers  appear  for  the  lateral  parts  of  the 
body.  The  presphenoid  likewise  develops  from  two  centers, 
which  are  apparent  in  the  ninth  week.  The  union  of  the 
presphenoid  with  the  basisphenoid  occurs  in  the  seventh  or 
eighth  month.  Each  greater  wing  develops  from  a  single 
center  of  ossification,  which  is  present  in  the  eighth  week. 
-The  process  of  ossification  spreads  from  this  center  to  produce 
not  only  the  greater  wing  but  also  the  external  pterygoid 
plate.  The  greater  wings  remain  separate  from  the  body 
until  some  time  during  the  first  year  after  birth.  Each  lesser 
wing  ossifies  from  a  center  that  appears  about  the  ninth 
week.  The  lesser  wings  unite  with  the  presphenoid  in  the 
sixth  fetal  month. 

The  internal  pterygoid  plate  diifers  from  the  other  parts  of 
the  sphenoid  in  that  it  does  not  ossify  in  cartilage  but  in 
membrane.  It  is  therefore  a  covering  hone.  Its  center  or 
centers  of  ossification  appear  in  the  fourth  month  in  the 
connective  tissue  in  the  lateral  walls  of  the  oropharyngeal 
cavity.  In  many  animals  this  plate  acquires  no  connection 
with  the  external  pterygoid  plate  but  remains  throughout 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   369 

life  a  distinct  bone,  the  pterygoid.  In  man  it  fuses  with 
the  external  plate  in  the  fifth  month. 

The  presphenoid  with  its  attached  lesser  wings,  and  the 
basisphenoid,  to  which  are  united  the  greater  wings  and  the 
pterygoid  plates,  remain  permanently  separate  bones  in  some 
animals.  In  man,  as  noted  above,  the  two  parts  of  the  body 
of  the  bone  unite  shortly  before  birth,  although  the  greater 
wings  remain  separate  until  some  months  after  that  event. 

The  ethmoid,  bone  and  the  inferior  turbinate  are  formed  in 
cartilage,  resulting  from  the  ossification  of  the  posterior  por- 
tion of  the  cartilaginous  nasal  capsule  (Fig.  168,  m).     The 


Fig.  168.— Cross-section  through  the  head  of  an  embryo  pig  3  cm.  (1.2  in.)  long, 
crown-rump  measurement.  The  nasal  cavities  are  seen  to  be  in  communication 
with  the  oral  cavity  at  the  places  designated  by  a  * :  K,  cartilage  of  the  nasal  sep- 
tum ;  m,  turbinal  cartilage ;  J,  organ  of  Jacobson  ;  J',  the  place  where  it  opens  intO' 
the  nasal  cavity ;  gf,  palatal  process ;  of,  maxillary  process ;  zl,  dental  ridge 
(Hertwig). 

latter  represents  the  anterior  extension  of  the  cartilaginous 
trabeculse  cranii  so  modified  as  to  constitute  a  receptacle  for 
the  olfactory  epithelium.  The  anterior  part  of  this  capsule- 
remains  cartilaginous  throughout  life  as  the  septal  and  lateral, 
cartilages  of  the  nose.  By  the  ossification  of  the  posterior 
part  of  the  nasal  capsule  the  ethmoid  and  the  inferior  tur- 
binate bones  are  produced.  Ossification,  beginning  in  the 
fifth  month,  involves  the  lower  and  the  middle  turbinals  and 
a  part  of  the  lateral  masses.  The  ossification  of  the  superior 
turbinal,  of  the  vertical  plate,  of  the  crista  galli,  and  of  the 

24 


370  TEXT-BOOK  OF  EMBRYOLOGY. 

remaining  parts  of  the  lateral  masses  is  effected  after  birth. 
The  bony  union  of  the  lateral  masses  with  the  median  plate 
is  completed  between  the  fifth  and  seventh  years. 

The  frontal  bone  is  a  covering  or  dermal  bone,  being  ossi- 
fied in  membrane  from  two  centers  of  ossification,  one  for 
each  lateral  half.  These  centers  are  situated  above  the 
orbital  arches  and  are  first  apparent  in  the  seventh  week.  At 
birth,  the  two  halves  of  the  bone  are  still  separate,  their 
union  not  occurring  until  during  the  first  year  of  life.  Some- 
times the  union  fails  to  take  place,  the  condition  of  the  per- 
sistent frontal  or  metopic  suture  being  known  as  metopism. 
Metopism  is  considerably  more  common  in  European  skulls 
than  in  those  of  lower  type. 

The  parietal  bone  is  also  ossified  in  membrane.  It  develops 
from  two  nuclei  which  soon  coalesce.  Their  position  con-e- 
sponds  to  that  of  the  future  parietal  eminence. 

The  bones  of  the  face  are  for  the  most  part  dermal  bones. 
Of  these,  the  upper  and  the  lower  maxillse  and  the  palate 
bones  belong  to  the  visceral-arch  skeleton.  The  others  de- 
velop in  the  membranous  wall  of  the  cranial  capsule. 

The  nasal  and  lacrimal  bones  ossify  each  from  a  single 
center,  which  appears  in  the  eighth  Aveek. 

The  malar  is  ossified  in  membrane  from  three  nuclei,  the 
process  beginning  in  the  eighth  week. 

The  palate  bone  is  formed  in  mucous  membrane  from  a 
single  center  which  is  situated  at  the  junction  of  the  vertical 
and  the  horizontal  plates. 

The  vomer  develops  from  two  centers  of  ossification  which 
appear  at  the  back  part  of  the  cartilaginous  nasal  septum. 
Each  center  gives  rise  to  a  lamina  of  bone,  the  two  laminae 
gradually  uniting  with  each  other  from  behind  forward,  and 
embracing  between  them  anteriorly  the  septal  cartilage. 

The  vomer  and  the  palate  bone  are  examples  of  the  forma- 
tion of  bone  in  mucous  membrane.  The  centers  of  ossifica- 
tion first  appear  in  the  eighth  week  in  each  case. 

The  skeleton  of  the  visceral  arches  includes  the  upper  and 
lower  maxillse,  the  hyoid  bone  with  a  part  of  the  styloid 
process,  the  ear  ossicles,  and  the  palate  bones.     The  palate 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   371 

bones  have  been  referred  to  above.  These  bones  of  the 
visceral-arch  skeleton  are  partly  primordial  and  partly  mem- 
branous. 

The  superior  maxilla  comprises  two  parts,  the  superior 
maxilla  proper  and  the  intermaxillary  bone.  While  these 
intimately  unite  in  man,  in  some  animals,  as  the  dog,  they 
are  permanently  distinct,  the  intermaxillary  bone  constituting 
the  important  and  conspicuous  premaxilla  of  the  dog.  The 
superior  maxilla  ossifies  in  membrane — within  the  mem- 
branous maxillary  process  of  the  first  visceral  arch — from 
an  uncertain  number  of  centers.  It  seems  probable  that 
there  are  three  nuclei  of  origin,  one  for  the  palate  process, 
one  for  the  malar  part  of  the  bone,  and  one  for  the  portion 
internal  to  the  infra-orbital  foramen  and  a  part  of  the  nasal 
wall.  The  formation  of  the  antrum  begins  in  the  fourth 
month  by  the  development  of  a  recess  or  fossa  on  the  inner 
or  nasal  wall  of  the  bone. 

The  palate  process  is  formed  by  the  growth,  on  the  inner 
aspect  of  the  bone,  of  a  shelf-like  projection  which  advances 
toward  the  median  line  until  it  meets  and  unites  with  its 
fellow  of  the  opposite  side  (Fig.  156).  The  horizontal  plate 
of  the  palate  bone  develops  similarly  and  very  shortly  after, 
and  thus  is  produced  the  hard  palate,  which  separates  the 
nasal  chambers  from  the  mouth.  The  two  halves  of  the 
hard  palate  unite  first  in  front,  their  union  being  completed 
by  the  twelfth  week.  If  union  is  incomplete,  the  anomaly 
of  cleft-palate  results.  The  intermaxillary  segment  begins 
its  development  in  the  seventh  or  eighth  week  upon  that 
part  of  the  nasofrontal  process  which  lies  between  the  nasal 
apertures.  In  the  fifth  month  the  intermaxillaries  fuse  with 
the  maxillae,  the  line  of  union  being  indicated  by  a  suture 
which  is  apparent  upon  the  oral  surface  of  the  palate  proc- 
esses. The  intermaxillaries  contain  the  germs  of  the  four 
incisor  teeth.  As  previously  mentioned,  deficiency  of  union 
between  the  maxilla  and  the  intermaxillary  results  in  the 
deformity  of  hare-lip.  Obviously,  the  hiatus  in  hare-lip 
will  be  found  to  be  not  median,  but  lateral,  corresponding  to 
the  position  of  the  line  of  normal  union. 


372 


TEXT-BOOK  OF  EMBRYOLOGY. 


The  lower  jaw  or  mandible  is  intimately  associated  in  its 
development  with  that  of  the  malleus  and  incus  of  the  middle 
ear.  Inasmuch  as  these  three  bones  are  diiFerentiated  from 
the  cartilaginous  and  membranous  visceral  skeleton  of  the 
first  visceral  arch  it  is  desirable  to  consider  their  develop- 
ment together. 

As  described  above,  the  membranous  jaw-arches  form  the 
lateral  and  lower  boundaries  of  the  mouth-cavity,  the  first 
visceral  arch  dividing  into  the  maxillary  process  and  the 
mandibular  arch.  There  appears  in  the  mandibular  arch  a 
bar  of  cartilage  Avhich  abuts  by  its  proximal  extremity  upon 
the  outer  wall  of  the  auditory  labyrinth.     This  cartilaginous 


Fig.  169. — Head  and  neck  of  a  human  embryo  eighteen  weeks  old  with  the 
visceral  skeleton  exposed  (after  Kolliker),  magnified.  The  lower  jaw  somewhat 
depressed  in  order  to  show  Meckel's  cartilage,  which  extends  to  the  malleus.  The 
tympanic  membrane  is  removed  and  the  annulus  tympanicus  is  visible:  Ao,  mal- 
leus, which  passes  uninterruptedly  into  Meckel's  cartilage,  Mk;  uk,  bony  lower 
jaw  (dentale),  with  its  condyloid  process  articulating  with  the  temporal  bone  ;  am, 
incus;  *<, 'stapes;  pr,  annulus  tympanicus;  grf,  processus  styloideus;  Isth,  liga- 
mentum  stylohyoideum  ;  kh,  lesser  cornu  of  the  hyoid  bone ;  gh,  its  greater  cornu. 


rod  segments  into  a  distal  portion,  Meckel's  cartilage  (Fig. 
169,  Mk),  and  a  smaller  proximal  piece,  which  is  called, 
in  comparative  anatomy,  the  palatoquadratiim.  From  the 
palatoquadratum    a    process,    the    palatopterygoid    process, 


the' DEVELOPMENT  OF  THE  HEAD  SKELETON.   373 

grows  toward  the  roof  of  the  mouth-cavity  and  becomes  a 
separate  segment.  The  piece  of  cartilage  remaining,  which 
represents  the  proximal  end  of  the  original  bar,  undergoes 
ossification,  becoming  the  incus  (Fig.  169,  am).  The  poste- 
rior or  proximal  extremity  of  Meckel's  cartilage,  becoming 
a  partly  separated  cartilage,  the  articulare,  ossifies  to  pro- 
duce the  malleus  (Fig.  169,  ha).  Though  the  form  of  the 
malleus  is  recognizable,  it  is  still  in  direct  continuity  with 
Meckel's  cartilage.  In  the  opposite  direction  it  is  articulated 
with  the  incus.  As  the  tympanic  ring  develops,  and  the  in- 
terval below,  between  this  ring  and  the  petrosa,  is  gradually 
narrow^ed  to  the  petrotympanic  or  Glaserian  fissure,  the  mal- 
leus comes  to  lie  within  the  tympanic  cavity,  being  continuous, 
through  the  fissure,  with  Meckel's  cartilage.  Upon  the  sepa- 
ration of  the  malleus  from  the  cartilage  of  Meckel,  the  long 
process  of  the  malleus  represents  the  former  bond  of  union 
and  therefore  occupies,  in  the  mature  state,  the  Glaserian 
fissure.  The  joint  between  the  malleus  and  the  incus  repre- 
sents the  primitive  vertebrate  jaw  articulation.  In  the  shark, 
for  example,  the  mandibular  joint  is  between  the  two  pieces 
into  which  the  cartilaginous  bar  of  the  first  visceral  arch 
divides — that  is,  between  the  palatoquadratum  and  the  repre- 
sentative of  Meckel's  cartilage,  the  mandibulare.  In  mam- 
mals, however,  the  malleus,  as  we  have  seen,  loses  its  con- 
nection with  the  mandible,  the  joint  between  the  latter  and 
the  skull,  the  temporomaxillary  articulation,  being  second- 
arily acquired  in  a  manner  to  be  pointed  out  hereafter. 
While  the  malleus  develops  for  the  most  part  by  ossification 
in  cartilage,  its  long  process  develops  in  membrane  as  a  small 
covering  or  dermal  bone,  the  angulare. 

The  membranous  lower  jaw  with  its  enclosed  bar  of  carti- 
lage becomes  osseous,  not  by  the  ossification  of  the  carti- 
lage, but  by  the  development  of  a  casing  of  bone  within  the 
surrounding  membrane.  In  other  words,  the  lower  jaw 
develops  chiefly  by  the  intramembranous  method  of  bone- 
formation.  Several  centers  of  ossification  appear,  and  from 
these  the  process  of  bone  production  extends  rapidly,  form- 
ing, by  the  fourth  month,  a  covering  or  dermal  bone,  the 


374  TEXT-BOOK  OF  EMBBYOLOOY. 

dentale  (Fig.  169,  ulS),  which  is  situated  mainly  on  the  outer 
side  of  Meckel's  cartilage.  A  smaller  plate  appears  on  the 
inner  side.  Thus  the  cartilage  comes  to  be  surrounded  by  an 
irregular  cylinder  of  bone.  The  cartilage  of  Meckel  plays 
a  comparatively  unimportant  part  in  the  ossification  of  the 
lower  jaw-bone  and  begins  to  degenerate  in  the  sixth  fetal 
month.  Its  distal  extremity,  however,  undergoes  ossifica- 
tion, thus  aiding  in  the  formation  of  a  small  part  of  the 
mandible  near  the  symphysis ;  while  a  posterior  segment, 
with  the  fibrous  tissue  encasing  it,  which  extends  from  the 
temporal  bone  to  the  inferior  dental  foramen,  persists  as  the 
internal  lateral  ligament  of  the  lower  jaw.  With  these 
exceptions,  Meckel's  cartilage  entirely  disappears.  The 
angle  of  the  mandible  and  a  small  part  of  the  ramus  are 
also  ossified  in  cartilage,  which  latter  is  developed  independ- 
ently of  Meckel's  cartilage.  From  the  posterior  part  of  the 
dentale  the  condyloid  process  develops  and  becomes  articu- 
lated with  the  glenoid  fossa  of  the  temporal  bone,  thus  estab- 
lishing the  temporomaxillary  articulation.  This  joint,  as  pre- 
viously stated,  is  a  secondary  one  and  replaces  in  mammals 
the  primitive  articulation  between  the  mandibulare  and  the 
palatoquadratum  of  the  lower  vertebrates. 

At  birtli,  the  two  lateral  halves  of  the  inferior  maxilla 
are  united  at  the  symphysis  by  fibrous  tissue ;  bony  union 
occurs  during  the  first  or  second  year  after  birth. 

To  summarize,  the  inferior  maxilla  develops  as  a  part  of 
the  visceral-arch  skeleton  and  is  chiefly  a  covering  bone,  since, 
with  the  exception  of  the  angle,  a  portion  of  the  ramus,  and 
a  small  part  near  the  symphysis,  which  are  of  cartilaginous 
origin,  it  is  formed  by  the  membranous  method  of  ossifica- 
tion. The  two  other  products  of  the  mandibular  arch,  the 
malleus  and  the  incus,  are  ossified  from  cartilage,  with  the 
exception  of  the  processus  gracilis  of  the  malleus,  which  is 
of  membranous  origin. 

The  development  of  the  hyoid  bone,  of  the  styloid  process  of 
the  temporal  bone,  and  of  the  stapes  was  referred  to  in  con- 
sidering the  cartilaginous  visceral-arch  skeleton,  but  for  the 
sake  of  clearness  and  completeness  it  may  not  be  amiss  to 


THE  DEVELOPMENT  OF  THE  HEAD  SKELETON.   375 

repeat,   in   this   connection,    some   points    previously   men- 
tioned. 

The  membranous  anterior  hyoid  or  second  visceral  arch,  at  a 
certain  stage  of  development,  presents,  in  its  interior,  the 
dorsoventral  cartilaginous  bar  known  as  Reichert's  carti- 
lage. This  is  parallel  with  Meckel's  cartilage,  and,  like  it,  is 
in  contact  by  its  dorsal  or  cranial  end  with  the  outer  wall  of 
the  auditory  labyrinth.  A  shorter  bar  of  cartilage  appears 
in  the  third  visceral  arch,  which  latter  is  known  also  as  the 
posterior  hyoid  arch.  Together,  these  two  cartilaginous  ele- 
ments furnish  the  stapes  of  the  middle  ear  and  the  hyoidean 
apparatus,  the  latter  consisting  of  the  hyoid  bone,  the  stylo- 
hyoid  ligaments,  and   the   styloid   processes.     In   man  the 


Fig.  170.— Hyoidean  apparatus  and  larynx  of  dog. 


hyoidean  apparatus  is  somewhat  rudimentary,  but  in  the  dog 
and  many  other  mammals  it  is  present  in  its  typical  form 
(Fig.  170).  In  such  animals  the  stylohyoid  ligament  of  hu- 
man anatomy  is  represented  by  a  bone,  the  epihyal,  the  hyoid 
bone  being,  therefore,  connected  w^ith  the  skull  by  a  series 
of  small  bones  articulated  with  each  other.  All  the  elements 
of  the  hyoidean  apparatus,  save  the  body  and  the  greater 
cornua  of  the  hyoid  bone,  are  produced  by  Reichert's  carti- 
lage ;  the  hyoid  body,  known  in  comparative  anatomy  as  the 
basihyal,  and  the  greater  cornua,  or  the    thyrohyals,  ossify 


376  TEXT-BOOK  OF  EMBRYOLOGY. 

from  the  cartilage  of  the  third  arch,  the  cartilage  for  the 
body  being  a  median  unpaired  segment  known  as  the  copula. 
Reichert's  cartilage  undergoes  division  into  five  segments. 
The  segment  at  the  cranial  end,  upon  ossification,  becomes 
the  stapes.^  This  ossicle,  by  the  closing  of  the  walls  of  the 
tympanic  cavity,  is  isolated  from  the  other  segments.  The 
second  piece,  the  tympanohyal,  ossifies  to  form  the  base  of  the 
styloid  process  and  ankyloses  firmly  with  the  temporal  bone 
at  the  point  of  junction  of  the  periotic  portion  of  that  bone 
with  its  tympanic  plate.  The  third  portion,  the  stylohyal, 
forms  the  lower  part  of  the  styloid  process.  It  undergoes 
ossification  later  than  the  tympanohyal  and  does  not  acquire 
osseous  union  with  it  until  the  time  of  adult  age.  It  some- 
times remains  separate  throughout  life.  The  fourth  seg- 
ment, the  epihyal,  does  not  even  become  cartilaginous  in 
man,  but  remains  fibrous,  constituting  the  stylohyoid  liga- 
ment. In  most  mammals  it  ossifies,  to  form  a  distinct  bone, 
the  epihyal.  The  ventral  extremity  of  the  cartilage  of 
Reichert,  the  ceratohyal,  produces  the  lesser  cornu  of  the 
hyoid  bone. 

THE  DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON. 

The  upper  and  lower  limbs  articulate  with  the  trunk 
through  the  medium  respectively  of  the  pectoral  and  pelvic 
girdles,  the  former  being  constituted  by  the  scapula  and  the 
clavicle,  and  the  latter  by  the  ossa  innominata.  As  in  the 
case  of  the  axial  skeleton,  the  bones  of  the  limbs  in  their 
development  pass  successively  through  a  membranous  and  a 
cartilaginous  stage. 

The  general  development  of  the  upper  and  lower  extremi- 
ties is  described  in  a  later  section.  As  stated  in  that  account, 
each  limb-bud  is  to  be  regarded  as  an  outgrowth  from  several 
primitive  segments,  the  tissue  composing  the  little  bud-like 
process  subsequently  differentiating  into  the  muscular,  carti- 
laginous, and  connective-tissue  elements  of  the  member. 
The  origin  of  each  limb  from  more  than  one  primitive  seg- 
ment has  been  established  chiefly  by  embryological  investi- 
'See  foot-note,  page  105. 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON     377 

gations  upon  the  lower  vertebrates,  and  is  borne  out  by  the 
fact  that  each  extremity  receives  its  nerve-supply  from  a 
series  of  spinal  nerves  instead  of  from  the  nerve-trunk  of 
any  one  segment. 

The  Development  of  the  Pectoral  and  the  Pelvic 
Girdles. — The  pectoral  or  shoulder  girdle  consists  in  its 
earliest  stage  of  a  pair  of  curved  bars  of  cartilage,  each  of 
which  is  made  up  of  a  dorsal  limb  occupying  approximately 
the  position  of  the  future  spine  of  the  scapula  and  approach- 
ing but  not  touching  the  spinal  column,  and  a  ventral  seg- 
ment lying  near  the  ventral  surface  of  the  trunk.  At  the 
angle  of  union  of  the  dorsal  and  ventral  parts  is  a  shallow 
depression,  an  articular  surface,  which  represents  the  point 
of  articulation  with  the  future  humerus. 

The  scapula  is  developed,  except  its  coracoid  process, 
from  the  dorsal  part  of  the  primitive  shoulder-girdle.  This 
soon  acquires  a  form  resembling  that  of  the  adult  scapula 
with  the  infraspinous  portion  of  the  bone  very  much  short- 
ened. Ossification  begins  at  the  neck  of  the  scapula  about 
the  eighth  week,  and  in  the  third  month  extends  into  the 
spine.  The  ventral  part  of  the  cartilaginous  shoulder-girdle 
extends  almost  to  the  median  line  of  the  chest-wall.  It 
divides  into  two  diverging  bars,  the  lower  one  of  which 
undergoes  ossification  in  birds  and  in  some  other  vertebrates 
to  form  the  conspicuous  coracoid  bone.  In  mammals,  how- 
ever, it  aborts  and  gives  rise  to  a  smaller  element,  the 
coracoid  process  of  the  scapula.  At  birth  the  human  scapula 
is  but  partially  ossified,  the  coracoid  process,  the  acromion, 
the  edges  of  the  spine,  the  base,  the  inferior  angle  and 
margins  of  the  glenoid  cavity  being-  cartilaginous.  The 
coracoid  process  ossifies  from  a  single  center  and  acquires 
osseous  union  with  the  body  of  the  bone  at  about  the  age  of 
puberty.  The  acromion  ossifies  from  two  or  three  nuclei 
and  joins  the  spine  between  the  twenty-second  and  twenty- 
fifth  years.  Still  other  centers  of  ossification  appear  from 
time  to  time.  Thus  there  is  an  accessory  center  for  the  base 
of  the  coracoid  and  the  adjacent  part  of  the  glenoid  cavity, 


378  TEXT-BOOK  OF  EMBRYOLOGY. 

and  one  at  the  inferior  angle  of  the  bone,  from  which  latter 
ossification  extends  along  the  vertebral  border. 

The  clavicle  does  not  develop  from  the  primitive  shoulder- 
girdle,  but  is  formed  in  membrane,  for  the  most  part,  as  a 
dermal  bone.  Its  ossification  begins  in  the  sixth  or  seventh 
week,  before  that  of  any  other  bone  in  the  body.  Subse- 
quently, cartilaginous  epiphyses  are  added,  one  at  each  end. 
It  is  by  means  of  the  epiphyses  that  the  bone  grows  in 
length. 

The  cartilaginous  pelvic  girdle  consists  of  a  pair  of  carti- 
lages, which  are  united  with  each  other  by  their  ventral 
extremities,  and  each  of  which,  by  its  dorsal  end,  is  articu- 
lated with  the  sacral  region  of  the  cartilaginous  spinal 
column.  At  about  the  middle  of  each  cartilage,  on  its  outer 
surface,  is  a  depression  representing  the  future  acetabular 
fossa.  Anterior  to  the  depression  is  a  large  aperture,  the 
thyroid  foramen,  the  upper  and  lower  boundaries  of  which 
are  respectively  the  pubic  and  ischiatic  rods  or  bars,  which 
make  up  the  ventral  portion  of  the  cartilage,  while  posterior 
to  the  fossa  is  the  iliac  segment,  which  has  a  somewhat 
irregular  plate-like  form.  Ossification  begins  in  the  third 
month,  proceeding  from  three  centers,  one  for  each  of  the 
three  divisions  of  the  innominate  bone.  At  the  time  of 
birth  a  large  proportion  of  the  original  cartilage  is  still 
•present,  the  os  pubis,  the  ischium,  and  the  ilium  being  sepa- 
rated from  each  other  up  to  the  age  of  puljerty  by  strips  of 
cartilage.  The  ischium  and  the  pubes  unite  first,  and  later 
acquire  osseous  union  with  the  ilium.  In  addition  to  the 
three  primary  centers  of  ossification,  other  and  secondary 
nuclei  appear  at  a  later  date  in  the  crest  of  the  ilium,  the 
tuberosity  of  the  ischium,  and  in  the  various  spines  and 
tubercles. 

The  skeleton  of  the  free  portions  of  each  extremity,  consist- 
ing at  first  of  a  continuous  mass  or  rod  of  partially  metamor- 
phosed mesenchymal  tissue,  undergoes  division  into  segments 
which  represent  the  skeleton  of  the  arm  or  of  the  thigh,  of 
the  forearm  or  of  the  leg,  and  of  the  hand  or  of  the  foot. 
This  segmentation  corresponds  with  that  of  the  entire  mass 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON.     379 

of  the  limb,  both  as  to  extent  and  order  of  appearance  (see 
page  380).  Nuclei  of  chondrification  now  appear,  one  in 
the  center  of  each  skeleton-piece,  from  which  cartilage  forma- 
tion extends  toward  either  end.  The  several  cartilaginous 
elements  thus  produced  present  approximately  the  respective 
forms  of  the  future  bones.  The  larger  cartilages  are  present 
in  the  upper  extremity  in  a  six  weeks'  embryo,  but  not  until 
somewhat  later  in  the  lower  limb.  All  the  bones  of  the 
extremities  are  of  endochondral  origin. 

The  long  bones  develop  in  a  fairly  uniform  manner.  The 
shaft  or  diaphysis  ossifies  from  a  single  center,  while  the  two 
epiphyses  each  present  several  centers.  The  centers  for  the 
diaphyses  appear  at  about  the  eighth  week,  ossification  pro- 
ceeding at  such  rate  that  at  birth  only  the  ends  of  the  long 
bones  are  cartilaginous.  The  centers  for  the  epiphyses  appear 
at  various  times  after  birth.  Osseous  union  between  the 
diaphysis  and  the  epiphyses  does  not  occur  until  the  growth 
in  length  of  the  bone  is  completed.  As  the  details  concern- 
ing the  time  of  appearance  and  the  number  of  these  centers 
are  to  be  found  in  the  text-books  of  anatomy,  they  are  omitted 
here. 

Each  "bone  of  the  carpus  and  of  the  tarsus  ossifies  from  a 
single  center,  except  the  os  calcis,  which  has  two  ossific 
nuclei.  The  bones  of  the  carpus  are  entirely  cartilaginous  at 
birth,  their  ossification  beginning  in  the  first  year  with  the 
appearance  of  a  center  in  the  scaphoid.  The  pisiform  bone 
is  the  last  of  the  series  to  ossify,  its  ossification  beginning  in 
the  twelfth  year. 

The  bones  of  the  tarsus  begin  to  ossify  earlier  than  those  of 
the  carpus.  The  os  calcis  and  the  astragalus  present  osseous 
nuclei  in  the  sixth  or  seventh  fetal  month,  and  the  cuboid 
shortly  before  birth.  With  these  exceptions  the  tarsal  bones 
undergo  ossification  between  the  first  and  the  fourth  years. 

The  metacarpal  and  the  metatarsal  bones  and  the  phalanges 
present  each  a  center  of  ossification  for  the  shaft  and  one 
epiphyseal  center.  In  the  case  of  the  phalanges  and  of  the 
metacarpal  bone  of  the  thumb  and  of  the  great  toe,  the  epi- 
physeal center   is  at   the  proximal  extremity,  while  in  the 


380  TEXT-BOOK  OF  EMBRYOLOGY. 

remaining  metatarsal  and  metacarpal  bones  it  is  at  the  distal 
end/  The  ossification  of  the  shaft  begins  in  the  eighth  or 
ninth  week  of  fetal  life ;  of  the  epiphyses,  not  until  several 
years  after  birth.  The  development  of  the  ungual  or  distal 
phalanges — of  the  hand,  at  least — is  peculiar  in  that  the 
ossification  begins  at  the  distal  extremity,  instead  of  in  the 
middle  of  the  shaft. 


THE  DEVELOPMENT  OF  THE   LIMBS. 

The  limbs  of  vertebrates  develop  from  little  bud-like 
processes  (Fig.  51)  that  spring  from  two  lateral  longitudinal 
ridges,  situated  one  on  each  side  of  the  body.  These  ridges 
are  not  exactly  parallel  with  the  median  plane  of  the  body, 
but  converge  somewhat  toward  that  plane  as  they  approach 
the  caudal  end  of'  the  embryo.  It  results  from  this  circum- 
stance that  the  posterior  limbs  are  placed  closer  together 
than  the  anterior.  In  man,  the  limb-buds  appear  soon  after 
the  third  week.  Each  bud  contains  a  basis  of  primitive  con- 
nective tissue  contributed  by  several  somites,  as  well  as  mus- 
cular structure,  which  is  the  offshoot  from  the  muscle-plates 
of  a  less  number  of  primitive  segments. 

The  assumption  of  the  origin  of  each  limb-bud  from  more 
than  one  primitive  segment  is  borne  out  by  the  nerve-supply 
of  the  fully-formed  limb,  each  extremity  being  innervated  by 
a  number  of  spinal  nerves  (compare  page  344).  The  con- 
nective tissue  of  the  limb-bud  produces  the  bony  structures 
of  the  limb,  while  the  outgrowths  from  the  muscle-plates 
contribute  their  musculature. 

In  the  fifth  week  each  limb-bud  becomes  divided,  by  a 
transverse  groove,  into  two  segments  (Fig.  48,  12,  13),  of 
which  the  distal  part  becomes  the  hand  or  foot,  while  the 
proximal  portion  very  soon  afterward  divides  into  the 
forearm  and  arm  or  leg  and  thigh.  Even  as  early  as  the 
thirty-second  day,  the  digitation  of  the  limb-buds — in  the 
case  of  the  upper  extremities — is  indicated  by  four  longi- 
tudinal parallel  lines  or  grooves  on  the  distal  extremity 
'  Quain's  Anatomy,  10th  edition. 


THE  DEVELOPMENT  OF  THE  LIMBS.  381 

of  each  (Fig.  48,  14).  By  the  conversion  of  these  grooves 
into  clefts,  the  fingers  appear,  in  the  sixth  week,  as  separate 
outgrowths.  The  development  of  the  upper  extremities  pre- 
cedes that  of  the  lower  by  twelve  or  fourteen  days,  so  that, 
when  the  fingers  are  present  as  distinct  projections,  the  toes  are 
just  being  marked  off  in  the  manner  noted  above  for  the 
fingers.  The  toes  begin  to  separate,  by  the  deepening  of  the 
intervening  clefts,  from  the  fiftieth  to  the  fifty-third  day.  By 
the  end  of  the  eighth  week,  the  fingers  are  perfectly  formed, 
with  the  exception  of  the  nails.  The  nails  have  their  beginning 
in  the  seventh  or  eighth  week,  in  little  claw-like  masses  of  epi- 
dermal cells,  which  are  attached  to  the  tips  of  the  digits 
instead  of  to  the  dorsal  surfaces.  Subsequent  transformations 
result  in  bringing  the  nail  into  its  normal  position  on  the 
dorsal  surface  of  the  distal  phalanx.  The  nails  are  well 
formed  by  the  fifth  month,  at  which  time  the  covering  of 
modified  epidermal  cells  begins  to  disappear.  The  extremity 
of  the  nail,  however,  does  not  break  through  so  as  to  project 
beyond  the  finger-tip  until  the  seventh  month.  A  more 
complete  account  of  the  development  of  the  nails  will  be 
found  in  connection  with  the  origin  of  the  skin  (page  247). 

The  Position  of  the  I/imbs. — The  paddle-like  limb- 
buds  at  first  project  laterally  almost  at  right  angles  with  the 
axis  of  the  trunk.  At  this  time  the  future  dorsal  surface  of 
each  limb  looks  toward  the  back  of  the  fetal  body  (dorsad), 
the  future  flexor  surface  toward  its  anterior  aspect  (ventrad), 
while  the  first  digits — the  future  thumb  and  great  toe — and 
consequently  the  radius  and  tibia,  occupy  the  side  of  the 
member  that  is  directed  headword  or  cephalad,  the  future 
little  finger  and  fifth  toe  with  the  ulna  and  fibula  looking 
caudad.  As  the  limbs  enlarge  and  differentiate  into  their 
respective  segments,  they  apply  themselves  to  the  ventral 
surface  of  the  body,  this  change  in  position  being  facilitated^ 
by  the  occurrence  of  the  future  elbow-  and  knee-flexions, 
which  cause  the  flexor  surfaces  of  the  forearm  and  leg,  re- 
spectively, to  approach  the  corresponding  surfaces  of  the 
upper  arm  and  thigh.  At  about  the  same  time,  the  distal 
segments,  the  hand  and  foot,  become  bent   in  the  opposite 


382  TEXT-BOOK  OF  EMBRYOLOGY. 

direction,  producing  the  condition  of  the  limbs  that  is  per- 
manent in  the  Amphibia — that  is,  the  condition  in  which  the 
dorsal  surface  of  the  proximal  segment  of  the  limb  faces  in 
the  same  direction  as  the  dorsal  surface  of  the  trunk,  while 
the  middle  segment  is  flexed  and  the  distal  is  extended.  To 
establish  the  permanent  condition  of  the  human  limbs,  there 
occur  an  outward  rotation  of  the  arms  and  an  inward  rotation 
of  the  lower  extremities,  on  their  long  axes.  The  thumb  and 
radius,  therefore,  instead  of  looking  cephalad,  are  now  di- 
rected dorsad — with  the  forearm  in  the  supine  position  and 
the  arm  outstretched — or  laterad,  away  from  the  median 
plane  of  the  body,  if  the  arm  hangs  by  the  side  in  the  ana- 
tomical position.  By  the  inward  rotation  of  the  lower  limb, 
the  great  toe  and  the  tibia  come  to  lie  toward  the  median 
plane  of  the  body,  causing  the  extensor  surface  to  look  ven- 
trad,  the  flexor  surface,  dorsad. 


TABULATED  CHRONOLOGY  OF  DEVELOPMENT. 


Maturation  of  ovum 
in  Graafian  follicle. 

Rupture  of  follicle. 

Entrance    of    ovum 
into  oviduct. 

Fertilization. 

STAGE  OF  THE  OVUM. 
First  Week.                               Second  Week. 

General 
Characters. 

Segmentation    of    fertilized 
ovum     to     form     morula 
while   passing    along  ovi- 
duct to  uterus. 

Cleavage-cavity         present, 
marking  stage  of  blastula. 

Great  increase  in  size. 

Cells  of  inner  cell-mass  re- 
arranged   to    form    ento- 
derm and  ectoderm. 

Outer   cells    become    thin- 
cells  of  Rauber. 

Embryonal  area. 

Primitive  streak. 

Mesoderm. 

(Amnion  completed   at   4th 
or  5th  day.     Peters.) 

Trophoblast  and  early  syn- 
cytium (3d  day). 

Ovum  in  uterus,  embedded 
in  mucosa. 

Amnion. 

Chorion  and  its  villi  (Fig. 
39).  Vascularization  of 
chorion  and  its  villi. 

Yolk-sac  partly  formed. 

Vascular 
System. 

Heart  indicated  as  two  tubes 
in  splanchnic  mesoderm. 

Vascular  system  repre- 
sented by  vascular  area 
of  yolk-sac. 

Digestive 
System. 

Oral  pit  (12th  or  14th  day). 
Gut-tract    partly    separated 
from  yolk-sac. 

Respiratory 
System. 

Genito-urinary 
System. 

Skin. 

Nervous 
System. 

Medullary  plate  (14th  day). 

Special  Sense 
Organs. 

Nasal  areas. 

Muscular 
System. 

Skeleton  and 
Limbs. 

383 


384  TEXT-BOOK  OF  EMBRYOLOGY. 

Tabulated  Chkonology  of  Development  {Continued). 


STAGE  OF  THE  EMBRYO. 
Third  Week.                           Fourth  Week. 

General 
Characters, 

Body  of  embryo  indicated. 
Dorsal  outline  concave. 
Vitelline  duct  (21st  day). 
Segmentation     of    paraxial 

mesoderm  begins. 
Visceral    arches   and   clefts 

begin  to  appear. 
Nasofrontal  process. 
Allantoic  stalk  (Fig.  46). 
Distinction  between  chorion 

leve     and     chorion    fron- 

dosum  begins. 

Marked  flexion  of  body  (21st 
to  2od  day) ;  gradual  un- 
coiling after  2od  day. 

Visceral  arches  and  yolk-sac 
attain  greatest  develop- 
ment (28th  day). 

Somites  well  formed. 

Well-marked  tail  (25th  day). 

Lining  cells  of  coelom  begin 
to  flatten. 

Increased  growth  of  allan- 
tois. 

Cephalic  flexures. 

Vascular 
System. 

Heart    with    single    cavity 
present,  soon  dividing  into 
atrium  and  ventricle. 

Vitelline  circulation  begun. 

Visceral-arch   vessels   begin 
to  appear. 

Division  of  atrium  begins. 
Completed  condition  of  vitel- 
line circulation. 
Allantoic  vessels  developing. 

Digestive 
System. 

Gut-tract  a  straight  tube  con- 
nected with  yolk-sac  by  a 
wide  aperture. 

Liver-evagination  present. 

Oral  pit  a  five-sided  fossa. 

Anal  plate. 

Alimentary    canal    presents 
pharvnx,  esophagus,  stom- 
ach, and  intestine. 

Pancreas  begun. 

Liver-diverticulum  divides. 

Bile-ducts  acquire  lumina. 

Pharyngeal  membrane 
breaks  down. 

Respiratory 
System. 

Pulmonary  anlage  as  a  longi- 
tudinal protrusion  of  ven- 
tral   wall    of     esophagus, 
afterward      becoming      a 
stalked  sac. 

Pulmonary     anlage     bifur- 
cates,   the    two    pouches 
being  connected  by  a  ped- 
icle, the  primitive  trachea, 
with  the  pharynx. 

Genito-urinary 
System. 

Wolffian  bodies  recognizable. 

Skin. 

Segmentation     of    paraxial 
mesoderm. 

Somites    or    primitive    seg- 
ments. 
Cutis-plate. 

Nervous 
System. 

Neural  canal :  its  cells  show 
differentiation    into   spon- 
gioblasts and  germ-cells. 

Fourth  ventricle  indicated. 

Fore-brain,    mid-brain,    and 
hind-brain    vesicles,    soon 
dividing  into  five  vesicles. 

Walls    of    cerebral    vesicles 

■  thicken. 

Ventral      roots     of     spinal 

nerves. 
Anterior  lobe  of  hypophysis 

begins. 

Special  Sense 
Organs. 

Auditory  pit  followed  by  otic 

vesicle. 
Olfactory  plates. 
Optic  vesicles  begin. 
Lens-vesicles. 

Otic   vesicle   with    recessus 

labyrinthi. 
Nasal  pits  distinct. 
Optic    vesicle    stalked    and 

transformed  into  optic  cup. 

Muscular 
System. 

Segmentation    of    paraxial 
mesoderm. 

Somites    or    primitive    seg- 
ments. 
Myotomes. 

Skeleton  and 
Limbs. 

Segmentation    of    paraxial 

mesoderm. 
Notochord. 

Somites  or  primitive  seg- 
ments. 

Skeletogenous  sheath  of 
chorda. 

Limb-buds  apparent  (about 
21st  day). 

TEXT-BOOK  OF  EMBRYOLOGY.  385 

Tabulated  Chronology  of  Development  {Continued). 


STAGE  OF  THE  FETUS. 
Fifth  Week.                                                  Sixth  Week. 

Body  shows  dorsal  concavity  in  neck- 
region. 

Globular  and  lateral  nasal  processes. 

Lacrimal  groove. 

Third  and  fourth  gill-clefts  disappear  in 
sinus  preecervicalis. 

Umbilical  cord  longer  and  more  spiral. 

Umbilical  vesicle  begins  to  shrink. 

Length  of  fetus  1  cm.  (§  inch). 

Nasofrontal,  lateral  nasal,  and  maxil- 
lary processes  unite. 
Umbilical  vesicle  shrunken. 
Amnion  larger. 

Primitive  aorta  divides  into  aorta  and 
pulmonary  artery. 

The  only  corpuscular  elements  of  the 
blood  during  the  first  month  are  the 
primitive  nucleated  red  blood-cells. 

Vitelline  circulation  atrophic  and  re- 
placed by  allantoic  circulation. 

Intestine  shows   flexures,  notably  the 
U-loop,  inaugurating  the  distinction 
between  large  and  small  bowel. 

Anal  pit. 

First  indication  of  teeth  in  the  form  of 
the  dental  shelf. 

Submaxillary  gland  indicated  by  epi- 
thelial outgrowth. 

Duodenum  well  formed;  caecum;  rec- 
tum (end  of  week). 

Right  and  left  bronchi  divide  into  three 
and  two  tubes  respectively  (5th  to  7th 
week). 

Larynx  indicated  as  dilatation  of  prox- 
imal end  of  trachea. 

Arytenoid  cartilages  indicated  (though 
not  cartilaginous). 

Thyroid  and  thymus  bodies  begun. 

Genital  ridges  appear  on  wall  of  body- 
cavity  and  soon  become  the  indiffer- 
ent genital  glands. 

Ducts  of  Miiller  appear. 

Genital  tubercle,  genital  folds,  and  gen- 
ital ridge  (external  genitals). 

Epidermis    present   as   two    layers   of 
cells. 

Cells  of  cutis-plate  proliferate  and  grad- 
ually spread  out  beneath  epidermis. 

Olfactory  lobe  begins. 

Arcuate  and  choroidal  fissures  on  me- 
sial surfaces  of  fore-brain  vesicles. 

Cells  of  central  canal  of  cord  ciliated. 

Ridge-like  thickening  of  roof  of  mid- 
brain. 

Membranes  of  brain  and  cord  indicated. 
Pineal  body  begins. 
Dorsal  root's  of  spinal  nerves. 
Some  tracts  of  spinal  cord  indicated, 
and  its  lumen  alters  (Fig.  123). 

Semicircular  canals  indicated. 
Eyes  begin  to  move  forward  from  side 
of  head. 

Semicircular  canals. 
Concha  of  external  ear. 
Outer  fibrous  and  middle  vascular  tu- 
nics of  eye. 
Eyelids. 

Mandibles  unite  (35th  day). 

Meckel's  cartilage. 

Limb-buds  segment. 

Digitation  indicated  (32d  day)  for  hand. 

Lower  jaw  begins  to  ossify. 

Clavicle  begins  to  ossify. 

Ribs  begin  to  chondrify. 

Bodies  of  vertebrae  are  cartilaginous. 

Fingers  as  separate  outgrowths. 

25 


386  TEXT-BOOK  OF  EMBRYOLOGY. 

Tabulated  Chronology  of  Development  {Continued ). 


1 

STAGE   OF   THE    FETUS.                                 { 
Seventh  Week.                         Eighth  Week. 

General 

Characters. 

Fetal  body  and  limbs  well 
defined  (Fig.  5o). 

Head  less  flexed. 

No  longer  any  trace  of  syn- 
cytium on  decidua  vera. 

Head  more  elevated  (Fig.  54). 
Free  tail  begins  to  disappear. 
Subcutaneous  lymph-vessels 

present. 
Cells  lining  the  coelom  are 

true  endothelium. 

Vascular 
System. 

Interventricular   septum  of 
heart  completed,  the  heart 
now  having  four  chambers. 

Other  corpuscular  elements 
added  to  blood  during  sec- 
ond month. 

Digestive 
System. 

Transverse    colon    and    de- 
scending colon  indicated. 

Parotid  gland  begins. 

True  endothelium  lines  the 
body-cavity. 

Gall-bladder      present      (2d  , 
month). 

Anlage  of  spleen   recogniz- 
able (2d  month). 

Respiratory 
System. 

Median  and  lateral  lobes  of 
thyroid  unite. 

Larynx  begins  to  chondrify. 
Formation    of    follicles    of 

thymus. 

Genito-urinary 
System. 

Maximum    development   of 
Wolffian  body. 

MuUerian  ducts  unite  with 
each  other.  Genital  groove. 

Bladder  present  as  spindle- 
shaped  dilatation  of  allan- 
tois. 

Suprarenal  bodies  recogniz- 
able. 

Skin. 

Nails  indicated  by  claw-like 
masses   of  epithelium   on 
dorsal  surfaces  of  digits. 

Corium  indicated  as  a  layer 
of    spindle-cells     beneath 
epidermis.       Development 
of  mammary  glands  begun. 

Nervous 
System. 

Fore-brain  vesicles  increase 
in  size  disproportionately. 
Cerebellum  indicated. 

Sympathetic  nerves  discern- 
ible. 

Special  Sense 
Organs. 

External  nose  definitely 
formed  (Fig.  155). 

Lens-capsule. 

Palpebral  conjunctiva  sepa- 
rates from  cornea. 

Muscular 
System. 

Muscles   begin  to  be  recog- 
nizable, though  not  having 
as  yet   the    characters  of 
muscular  tissue. 

Skeleton  and 
Limbs. 

Ossific  centers  for  vertebral 
arches   and    for   vertebral 
bodies ;  ossific  centers  for 
frontal  bone  and  for  squa- 
mosa. 

Membranous  primordial  cra- 
nium begins  to  chondrify. 

Claw-like  anlages  of  nails. 

Ribs  begin  to  chondrify.  Cen- 
ters of  ossification  of  basi- 
sphenoid,  of  greater  wings, 
of    nasal     and     lacrimal 
bones,  of  malar,  vomer,  pal- 
ate, neck  of  scapula,  diaph- 
yses  of  long  bones  and  of 
metacarpal  bones.   Fingers 
perfectly  formed.    Toes  be- 
gin to  separate  (53d  day). 

TEXT-BOOK  OF  EMBRYOLOGY.  387 

Tabulated  Chronoi,ogy  of  Development  {Continued). 


STAGE   OF   THE    FETUS. 
Ninth  Week.                                              Thied  Month. 

Weight,  15  to  20  grams;  length,  25  to  30 

mm.  (1  to  If  inches). 
Hard  palate  completed. 
Free  tail  has  disappeared. 
Differentiation  of  lymph-nodes  begins 

(0.  Schultze).    Cloaca  divided. 

Weight  (end  ofmonth),  4  ounces;  length, 
2|  inches. 

At  first  chorion  leve  and  chorion  fron- 1 
dosum  present;    later,  formation   of 
placenta  (see  second  frontispiece). 

Pericardium  indicated. 

Placental  system  of  vessels. 
Blood-vessels  penetrate  spleen. 

Anal  canal  formed  by  division  of  cloaca. 
(Anus  opens  at  end  of  2d  month,  ac- 
cording to  Tourneux.) 

Mouth-cavity  divided  from  nose  (end  of  j 
month).    Soft  palate  completed  (11th 
week).    Papillae  of  tongue.    Evagina- 
tion  for  tonsil.    Intestine  begins  to  re- 
cede within  abdomen  (10th  week).   Ro- 
tation of  stomach.    Vermiform  appen- 
dix as  a  slender  tube.    Omental  bursa. 
Gastric  glands  and  glands  and  villi  of 
intestine    fairly    well    formed     (10th 
week).    Liver  very  large.    Peritoneum 
has  its  adult  histological  characters. 

Epiglottis. 

External    genitals    begin  to  show  dis- 
tinctions of  sex. 

Ovary  and  testis  distinguishable  from 
each  other. 

Kidney  has  its  characteristic  features. 

Urogenital  sinus  acquires  its  own  aper- 
ture by  division  of  cloaca. 

Union  of  testis  with  canals  of  Wolffian 

body  complete. 
Testes  in  false  pelvis. 
Ovaries  descend. 
Prostate  begun  (12th  week). 

Corium  proper  present  as  distinct  layer. 
Nails  not  quite  perfectly  formed. 
Beginning  of  development  of  hair  as 
solid  ingrowths  of  epithelium. 

Corpus  striatum  indicated. 
Corpora  quadrigemina  represented  by 
two  elevations  on  mid-brain  roof. 

Cerebrum   covers   inter-brain.     Fornix 
and  corpus  callosum  begun.    Fissure 
of  Sylvius.    Calcarine  fissure.    Crura 
cerebri.    Restiform  bodies.    Pons. 

External  ear  indicated  (Fig.  154). 
Ciliary  processes  indicated. 

Eyes  nearly  in  normal  position. 
Eyelids  begin  to  adhere  to  each  other. 

• 

Centers  of  ossification  of  presphenoid, 
of  lesser  wings  of  sphenoid,  and  of 
shafts  of  metatarsal  bones. 

Beginning  ossification  of  occipital  bone, 
of  tympanic,  of  spine  of  scapula,  of 
ossa  innominata. 

Cartilaginous  arches  of  vertebrae  close. 

Limbs  have  definite  shape ;  nails  almost 
perfectly  formed. 

388  TEXT-BOOK  OF  EMBRYOLOGY. 

Tabulated  Chronology  of  Development  {Continued). 


STAGE  OF   THE  FETUS. 
Fourth  Month.                          Fifth  Month. 

General 

Characters. 

Weight,  7|  ounces ;  length,  5 
inches. 

Head  constitutes  about  one- 
quarter  of  entire  body. 

Weight,  1  lb.;  length,  8  in. 
Active  fetal  movements  be- 
gin. Two  layers  of  decidua 
coalesce,  obliterating  the 
space  between  vera  and  re- 
flexa.  Lymphatic  glands 
begin  to  appear. 

Vascular 
System. 

Heart  very  large. 

Digestive 
System. 

Enamel  and  dentine  of  milk- 
teeth.  Germs  of  permanent 
teeth  (17th  wk) ;  (for  1st  mo- 
lar, 16th  wk).    Muscularis 
(longitudinal  and  circular) 
of  stomach  and  esophagus. 
Intestine    entirely  within 
abdomen.     Acid   cells    of 
peptic  glands.    Malpighian 
bodies    of   spleen.      Anal 
membrane  disappears. 

Salivary  glands  acquire  lu- 

mina. 
Villi  of  large  intestine  begin 

to  disappear. 
Liver  very  large. 
Meconium  shows   traces  of 

bile    (sometimes   early   in 

fourth  month). 

Respiratory 
System. 

Cells  of  tracheal  and  bron- 
chial   mucous    membrane 
ciliated. 

Genito-urinary 
System. 

■Sexual  distinctions  of  exter- 
nal   organs    well  marked. 
Closure  of  genital  furrow. 
Scrotum.     Prepuce.    Pros- 
tate well  formed. 

Distinction   between  uterus 

and  vagina. 
Hymen  begins. 

SMn. 

Papillae  of  corium.    Subcuta- 
neous fat  first  appears.  La- 
nugo or  embryonal  down 
on  scalp  and  some  other 
parts. 

Panniculus  adiposus. 
Lanugo  more  abundant. 
Sebaceous  and  sweat-glands 
begin. 

Nervous 
System. 

Parieto-occipital  fissure. 

Corpora  albicantia. 

Transverse  fibers  of  pons. 

Middle  peduncles  and  chief 
fissures  of  cerebellum. 

Spinal  cord  ends  at  9nd  of 
coccyx. 

Deposit  of  myelin  on  fibers 
of  posterior  roots,  extend- 
ing to  Burdach  and  Goll. 

Fissure  of  Rolando.  Body  of 
fornix  and  corp.  callosum. 
Longitudinal  fibers  in  cru- 
ra cerebri.  Superior  pedun- 
cles. Anterior  pyramids  of 
medulla.  Chief  transverse 
fissures  of  lateral  lobes  of 
cerebellum.  Deposit  of  my- 
elin completed  for  tract  of 
Goll  and  later  of  Burdach, 
and  for  short  commissural 
fibers  (Tourneux). 

Special  Sense 
Organs. 

Eyelids  and  nostrils  closed. 
Cartilage  of  Eustachian  tube. 

Organ  of  Corti  indicated. 

Muscular 
System. 

Differentiation  of  muscular 
tissue  of  arms. 

Skeleton  and 
Limbs. 

1 

Osseous  center  for  internal 

pterygoid  plate. 
Antrum  of  Highmore  begins. 
Ossification  of  malleus  and 

incus. 
Tympanic  ring. 

Ossification  of  .stapes  and  pe- 
trosa.  Opisthotic  and  pro- 
otic  appear.  Ossification 
begins  in  middle  and  infe- 
rior turliinnls  and  lateral 
mnsscs  of  ethmoid.  Inter- 
niil  jitcrygoid  plate  fuses 
witli  external.  Intermax- 
ilhiries  fuse  with  maxilla. 
Leg.s  longer  than  arms. 

.     TEXT-BOOK  OF  EMBRYOLOGY.  389 

Tabulated  Chronology  of  Development  {Continued). 


STAGE  OP   THE  FETUS. 
Sixth  Month.                                            Seventh  Month. 

Weight,  2  pounds ;  length,  12  inches. 
Vernix  easeosa  begins  to  appear. 
Amnion  reaches  maximum  size  ;  amni- 
otic fluid  of  maximum  quantity. 

Weight,  3  pounds ;  length,  14  inches. 
Surface  less  wrinkled  owing  to  increase 
of  fat. 

Peyer's  patches. 

Trypsin  in  pancreatic   secretion  (fifth 
or  sixth  montli). 

Meconium  in  large  intestine. 
Ascending  colon  partly  formed. 
Caecum  below  right  kidney. 

Air-vesicles  of  lungs  begin  to  appear. 

Walls  of  uterus  thicken. 

Testes  at  internal  rings  or  in  inguinal 

canals. 

Vernix  easeosa  begins  to  appear. 
Eyebrows  and  eyelashes  begin. 

Epithelial  buds  for  sebaceous  glands  ac- 
quire lumina.    Branching  of  cords  of 
milk-glands.     Eponychium   of  nails 
lost ;    nails    said   to    break   through. 
Lanugo  over  entire  body. 

Collateral  and  calloso-marginal  fissures. 

Body  of  fornix    and  corpus  callosum 
complete. 

Hemispheres  of  cerebrum  cover  mid- 
brain. 

Cerebral  convolutions  more  apparent. 

Corpora  quadrigemina. 

Myelination  of  fibers  of  direct  cerebellar 

tracts.    (Crossed  pyramidal  tracts  not 

until  after  birth.) 

Lobule  of  ear  more  characteristic. 

Lens-capsule  begins  to   acquire  trans- 
parency.   Eyelids  permanently  open. 
Pupillary  membrane  atrophies. 

Differentiation  of   muscular   tissue  of  j 
lower  extremities. 

Lesser  wings  unite  with  presphenoid. 
Meckel's  cartilage  begins  to  retrograde. 
Ossiflc  nuclei  of  os  calcis  and  astragalus. 

Basisphenoid   and    presphenoid    unite 
(7th  or  8th  month). 

o90  TEXT-BOOK  OF  EMBRYOLOGY. 

Tabulated  Chronology  of  Development  {Concluded). 


STAGE   OF  THE   FETUS. 

Eighth  Month.                         Ninth  Month. 

General 

Characters. 

Weight,  4  to  5  pounds ;  length, 

16  inches. 
Body  more  plump. 

Weight,  6  to  7  pounds ;  length, 

20  inches. 
Umbilicus  almost  exactly  in 

middle  of  body. 

Vascular 
System. 

Digestive 
System. 

Ascending  colon  longer. 
Caecum  below  crest  of  ilium. 

Meconium  dark  greenish. 

Eespiratory 
System. 

Genito-urinary 
System. 

Testes  in  inguinal  canals. 

Testes  in  scrotum. 
Labia  majora  in  contact. 

Skin. 

Vernix  caseosa  covers  entire 
body. 

Skin  brighter  color 

Lanugo  begins  to  disappear. 

Nails  project  beyond  finger- 
tips. 

Increase     of    subcutaneous 
fat. 

Lanugo  almost  entirely  ab- 
sent. 

Galactopherous  ducts  of 
milk-glands  acquire  lu- 
mina. 

Nervous 
System. 

Spinal  cord  ends  at  last  lum- 
bar vertebra. 

Special  Sense 
Organs. 

Ossification  of  bony  lamina 
spiralis  and  of  modiolus. 

Neuro-epithelial  layer  of  re- 
tina completed ;  macula 
still  absent. 

Choroidal  fissure  closes. 

1  Muscular 
System. 

Skeleton  and 
Limbs. 

Ossification  in  lower  epiph- 
ysis of  femur,  sometimes 
also  in  upper  epiphyses  of 
tibia  and  humerus. 

Tympanohyal  begins  to  os- 
sify. 

Ossific  nuclei  for  body  and 
great  horn  of  hyoid  bone. 

INDEX. 


Abdominal  cavity,  development  of, 

198 
Accessory  thyroid,  209 
Acetabular  fossa,  378 
Achoria,  84 
Achromatin,  25 
Acid  cells,  formation  of,  189 
Acoustic  ganglion,  297 
Acusticofacial  ganglion,  297 
Adamantoblasts,  127 
Adenoid  tissue,  development  of,  117 
Adipose  tissue,  formation  of,  115 
After-birth,  94 
After-brain,  263 
Age  of  fetus,  estimation  of.  111 
Air-chamber  of  hen's  egg,  27 
Air-sacs,  development  of,  207 
Alfe  of  nose,  development  of,  338 
Allantoic  arteries,  81,  148 
circulation,  81 

formation  of,  148 
stalk,  76 
veins,  81,  148 
Allantois,  80,  173,  232 
function  of,  81 
respiratory  function  of,  183 
Alar  lamina,  266 
Alecithal  ova,  24 

Alimentary   canal,    development   of, 
168 
diflferentiation  into  separate  re- 
gions, 180 
histological  alterations  in,  188 
tract,    alteration     in     position    of 
parts,  185 
increase  in  length  of,  184 
Alveoli,  pulmonary,  development  of, 

207 
Ameloblasts,  127 
Amnion,  73,74 
false,  73 
of  man,  76 
Amnion-fold,  72,  73,  75 
Amniota,  75 
Amniotic  cavity,  76,  77 
fluid,  76,  77 

function  of,  77 
suture,  75 
Amphibians,  blastula  of,  46 
Amphioxus,  blastula  of,  45 
skeletal  apparatus  of,  347 
Ampullae  of   semicircular  canals,  de- 
velopment of,  324 


Ampullae,  seminal,  223 
Anal  canal,  233 

membrane,  178 

plate,  178 
Anamnia,  75 
Animal  pole,  25 
Animalculi'sts,  18 
Anlage,  159 
Annular  sinus,  163 
Anomalous    arrangements    of    aortie 

arch,  152 
Anterior  chamber  of  eye,  318 

nares,  development  of,  134,  336 

pyramidal  tracts  of  medulla,  devel- 
opment of,  266 
Antitragus,  formation  of,  333 
Antrum"  of  Highmore,  development 

of,  337 
Anus,  development  of,  178 

imperforate,  180 
Aorta,  caudal,  150 

development  of,  144 

primitive,  137,  150 
Aortic  arch,  anomalous  arrangements 
of,  152 

arches,  149 
Appendages  of  skin,  247 
Appendicular  skeleton,  347 

development  of,  376 
Aqueduct  of  Sylvius,  development  of, 

271 
Arch,  hyoid,  104 

mandibular,  104 

maxillary,  104 

of  aorta,  development  of,  152 
Archenteron,  47,  49 
Arches,  aortic,  149 

branchial,  103 

mandibular,  123 

visceral,  101 
Archiblast,  58 
Arcuate  fissure,  282,  283 
Area,  embryonal,  50 

glandular,  251 

opaca,  51 

pellucida,  51 

vasculosa,  51,  79,  136 
Areas,  nasal,  133,  334 
Areola,  development  of,  252 
Areolar  tissue,  development  of,  114 
Arrectores  pilorum,  246 
Arteria  centralis  retinse,  development 
of,  311 

391 


392 


INDEX. 


Arterial  system,  feta-l,  149 
Arteries,  allantoic.  81,  148 
umbilical.  93,  149 
vitelline,  137 
Artery,    carotid,    common,    develop- 
ment of,  151 
external,  development  of,  151 
internal,  development  of,  151 
innominate,  development  of,  152 
middle  sacral,  development  of,  150 
pulmonary,  development  of,  152 
subclavian,    left,  development    of, 
152 
right,  development  of,  151 
superior  vesical,  166 
Arytenoid  cartilages,  development  of, 

207 
Ascending  colon,  formation  of,  186 
mesocolon,  formation  of,  186 
root  of  fifth  nerve,  206 
root  of  vagus,  266 
Aster,  41 

Atlas,  formation  of,  355 
Atresia  of  pupil,  314 
Atrioventricular  canal,  141 

valves,  142 
Atrophic  tubules  of  Wolffian  body, 

216 
Attraction-sphere,  41 
Auditory  apparatus,  development  of, 
321 
meatus,  external,  formation  of,  332 
nerve,  formation  of,  297 
nucleus,  lateral  accessory,  297 
pit,  322 
Auricle,  development  of,  333 
Auricles,  division  into  right  and  left, 

142 
Auricular  appendages,  144 

canal,  141 
Auriculoventricular  apertures,  145 

valves,  146 
Axial  fiber  of  spermatozoon,  20 
skeleton,  347 

development  of,  348 
Axis,  development  of,  354 
Axis-cylinder  process,  260 

Baetholin,  glands  of,  237 
Basal  ganglia,  279,  280 

lamina.  266 
Basi-occipital  bone,  364 
Basisphenoid,  368 
P.elly-stalk,  76 
Bifid  uterus,  2.30 

Bile-capillaries,  formation  of,  192 
Bile-ducts,  formation  of,  192 
Bladder,  development  of,  232 
Blastopore,  47 
Blastula,  stage  of,  45 
Blood,  development  of,  115,  1.35 
Blood -corpuscles,  red,  primitive,  136 
Blood-islands,  of  Pander,  135 
Blood-lacunse,  87 
"Blue  baby,"  143 


Bodies,  polar,  31 

Body  of  vertebra,  formation  of,  352 
Body-cavity,  55,  58,  197 
Body-wall,   development  of   muscles 
of,  343 

formation  of,  71 
Bony  cochlea,  development  of,  327 

labyrinth,  development  of,  326 

semicircular  canals,  327 
Bowman,  capsule  of,  216,  217 
Brain,  development  of,  262 
Brain-case,  358 
Brain-membranes,    development    of. 

278 
Brain-vesicles,  262 

derivatives  of,  292 
Branchial  arches,  103 

development  of,  344 
Bridge  of  nose,  development  of,  338 
Brunner,  glands  of,  189 
Bulbus  arteriosus,  142 

vestibuli,  237 
Burdach,  tract  of,  myelination  of,  388 
Bursa,  omental,  187,  200 

pharyngeal,  124 
Bursal  sacs,  development  of,  115 

Caducous  membranes,  85 
Csecum,  development  of,  185,  186 
Calcar  avis,  284 
Calcarine  fissure,  279,  284 
Callosomargiual  fissure,  285 
Calyces  of  kidney,  formation  of,  218 
Canal,  anal,  233 

atrioventricular,  141 

auricular,  141 

hyaloid,  315 

niedullarv,  62 

neural,  62,  2.55,257 

neurenteric,  66,  257 

of  anus,  180 

of  His,  133,  208 

of  Nuck,  232 

of  Stilling,  315 
Canaliculi,  lacrimal,  development  of, 

320 
Canalis  reuniens,  324 
Capsule  of  Bowman,  216,  217 
Cardinal  veins,  148,  154 
Carotid  artery,  common,  development 
of,  151 
external,  development  of,  151 
internal,  development  of,  151 
Carpus,  development  of  bones  of,  379 
Cartilage,  formation  of,  115 

Meckel's,  104,  372 

Reichert's,  104 
Cartilage-cells,  115 
Cartilaginous  capsule  of  cochlea,  328 

cranium,  360 

ear-capsule,  326 

ribs,  356 

sheatli  of  spinal  cord,  353 

stage  of  skeleton,  .348 
of  trunk  skeleton,  351 


INDEX. 


393 


■Caudal  aorta,  150 
Cavity,  amuiotic,  76 

pleuroperitoneal,  58 

segmentation,  45 
Cell-mass,  inner,  45 

intermediate,  69 

outer,  45 
Cells,  sexual,  29 

mesenchymal,  58 
Cementum  of  tooth,  125 

development  of,  129 
Central  canal  of  cord,  formation  of, 
262 

lobe,  formation  of,  281 
Centrolecithal  ova,  25 
Centrosome,  41 
Cephalic  flexure,  101,  264 

ganglia,  development  of,  296 
Ceratohyal,  376 

Cerebellum,  development  of,  268 
Cerebral  fissures,  development  of,  278 

vesicles,  262,  263 
Ceruminous  glands,  250 
Cervical  fistula,  105 

flexure,  101 

rib,  354,  357 
Chalazse,  27 
Chambers  of  eye,  318 
Chorda  dorsalis,  65 
formation  of,  348 

stage  of,  348 
Chordae  tendinese,  146 
Chordal  epithelium,  349 

plate,  66 

region  of  primitive  skull,  361 
Choriata,  84 
Choriocapillaris,  316 
Chorion,  82 

frondosum,  84 

leve,  83 

primitive,  82 

true,  82 
Choroid,  coloboma  of,  316 

development  of,  316 

fissure,  282,  283 

plexus,  284 

plexuses  of  fourth  ventricle,  267 
Choroidal  fissure,  306,  316 
Chromatin  substance,  25 
Cicatricula,  26 
Ciliary  body,  development  of,  316 

ganglion,  296 

muscle,  development  of,  316 

processes,  development  of,  309,  316 
Circulation,  allantoic,  81,  148 

placental,  135 

portal,  161 

vitelline,  formation  of,  135 
Claustrum,  279 
Clavicle,  development  of,  378 
Cleavage,  kinds  of,  43 

of  ovum,  41 

partial  discoidal,  44 
peripheral,  44 

total  equal,  43 


Cleavage,  total  unequal,  43 
Cleavage-cavity,  45 
Cleavage-nucleus,  39 
Cleavage-planes,  42 
Cleft  palate,  formation  of,  125 

sternum,  357 
cause  of,  74 

uvula,  formation  of,  125 
Clefts,  visceral,  101 
Climacteric,  35 

Clitoris,  development  of,  235,  236 
Cloaca,  173,  179,  233 
Cloacal  depression,  180,  233 
Closing  membrane,  102,  106,  177 
Coccygeal  curve,  101 

vertebrse,  ossification  of,  356 
Cochlea,  bony,  development  of,  327 
Cochlear  duct,  formation  of,  323 

ganglion,  297 

nerve,  329 
Ccelenteron,  47,  49 
Coelom,  55,  58,  197 
Collateral  fissUre,  279,  284 
Coloboma  of  choroid,  316 

of  iris,  318 
Colon,  ascending,  formation  of,  186 

descending,  formation  of,  184,  186 

transverse,  formation  of,  186 
Columnse  carnese,  140 
Commissures  of  brain,  development 
of,  279 

of  cord,  white,  260 
Conarium,  274 

modifications  of,  274 
Cone-visual  cells,  307 
Congenital  atresia  of  pupil,  314 

diaphragmatic  hernia,  161 

fecal  fistula,  190 

hernia,  226 

umbilical  hernia,  188 
Coni  vasculosi,  formation  of,  223 
Connective    tissues,   development  of, 

113 
Constructive     stage     of     menstrual 

cycle.  35 
Copula  of  hyoid  bone,  363 
Coracoid  bone,  377 

process  of  scapula,  377 
Cord,  spinal,  development  of,  257 

umbilical,  92 
Cords  of  cells,  135 
Corium,  development  of,  245 
Cornea,  development  of,  315 
Corona  radiata,  23,  29 
Coronarv  ligament,  193 
of  liver.  202 

sinus  of  heart,  156 

valve,  144 
Corpora  albicantia,  272 

bigemina,  271 

cavernosa,  formation  of,  238 

quadrigemina.  271 
Corpus  callosum,  formation   of,   285, 
287 

hemorrhagicum,  33 


394 


INDEX. 


Corpus  luteum  of,  pregnancy,  33,  34 
false,  34 

of  menstruation,  34 
true,  34 

spongiosum,  formation  of,  239 

striatum,  279 
Corpuscle  of  Hassal,  210,  211 
Corti,  organ  of,  325 
Costal  process  of  vertebra,  formation 

of,  352,  356 
Cotyledons  of  placenta,  89 
Covering  bones,  359 
Cowper,  glands  of,  239 
Cranial  capsule,  358 

nerve-fibers,  development  of,  296 
Cranium,  cartilaginous,  360 

membranous,  359 

osseous,  363 
Cristas  acusticse,  325 
Crossed  pvramidal  tract,  myelination 

of,  389 
Crura  cerebri,  development  of,  270 
Crusta  petrosa,  129 
Cryptorchism,  226 

Crvstalline  lens,  development  of,  312 
Cutis-plate,  69,  245,  341 
Cuvier,  duct  of,  148,  154,  160 
Cystic  duct,  development  of,  192 

Daughter -CELLS,  22 
Daughter-wreaths,  41 
Decidua  menstrualis,  36,  85 

of  pregnancy,  86 

reflexa,  86 

serotina,  86,  91 

vera,  86 
Deciduse,  85 
Dendrites,  260 
Dental  groove,  126 

papilla,  127,  128 

processes,  129 

ridge,  125 

shelf,  125 
Den  tale,  374 
Dentate  fissure,  279,  283 
Dentinal  fibers,  129 

tubules,  129 
Dentine,  125 
Dermal  bones,  359 

navel,  74 
Descending  colon,  formation  of,  186 
Descent  of  testicles,  225 
Destructive  stage  of  menstrual  cycle, 

36 
Deutoplasm  of  hen's  egg,  26 

of  ovum,  24 
Development  during  eighth  mouth, 
111 

during  eighth  week,  108 

during  fifth  month,  110 

during  fifth  week,  107 

during  ninth  month,  111 

during  second  month,  107 

during  seventh  month,  110 

during  sixth  month,  110 


Development  during  third  month,  109 

during  third  week,  106 

length  of  time  necessary  for,  18 

tabulated  chronology  of,  383 

theories  of,  17 
Diaphragm,  development  of,  161 
Diaphragmatic     hernia,    congenital, 
161 

ligament,  225 
Digestive    svstem,    development    of, 

168, 385-390 
Digitation  of  limb-buds,  380 
Diphyodont,  125 
Direct  cerebellar  tract,  myelination 

of,  385 
Discoidal  cleavage,  partial,  44 
Discus  proligerus,  29,  228 
Disk,  germinative,  26 
Dorsal  curve,  101 

mesentery,  173 
Double  monster,  origin  of,  51 

uterus  230 
Duct  of  Cuvier,  148,  154,  160    ■ 

mesonephric,  214 

of  Gartner,  231 

of  Miiller,  220,  224,  230,  242 

of  Rathke,  225 

pronephric,  212 

segmental,  213 

thyroglossal,  133,  208 

thyroid,  208 

vitelline,  72,  78,  169 

Wolffian,  214 
Ductus  Arantii,  164 

communis  choledochus,  formation 
of,  192 

endolymphaticus,  323 

venosus,  93,  164 
Duodenum,  formation  of,  199 

Ear,  external,  development  of,  331, 
333 

internal,  development  of,  321 

middle,  development  of,  331 
Ear-capsule,  cartilaginous,  326 
Ectoderm,  47,  48 

derivatives  of,  59 
Egg,  ultimate  origin  of,  29 
Egg-columns,  29,  227 
Egg-envelopes,  23 
Egg- pi  asm,  24 
Egg-tubes,  primary,  29 
Eighth  month,  development  during, 
111,  390 

pair   cranial    nerves,    development 
of,  299 

week,  development  during,  108,  386 
Ejaculatory  duct,  formation  of,  224 
Elastic  tissue,  formation  of,  114 
Eleventh  pair  of  cranial  nerves,  300 
Embedding  of  ovum,  86 
Embryo,  difierentiation  of,  61 

of  eight-and-a-half  weeks,  110 

of  fifteenth  day,  98 

of  six  weeks,  107 


INDEX. 


395 


Embryo  of  thirteenth  day,  97 

of  three  weeks,  101 

of  twenty-eight  days,  105 

segmentation  of  body  of,  69 

stage  of,  19,  96 
Embryology,  defined,  17 
Embryonal  area,  50 

down,  250 
Embryonic  crescent,  51 
Eminentia  collateralis,  285 
Enamel  of  milk  teeth,  formation  of, 
128 

of  teeth,  125 
Enamel-cells,  127 
Enamel-germ,  primitive,  126 
Enamel-germs  of   permanent  teeth, 

129 
Enamel-prisms,  127 
Enamel-sac,  126 
Endocardium,  140 
Endochondral  bones,  359 
Endolymph,  331 
Eudoskeleton,  347 
Endothelium,  formation  of,  58,  115 
EnteroccBl,  55 
Entoderm,  47,  48 

derivatives  of,  59 
Epencephalon,  268 
Ependyma,  286 
Ependymal  layer,  260 
Epiblast,  47 

Epidermis,  development  of,  245,  246 
Epididymis,  formation  of,  223 

head  of,  223 
Epigenesis,  doctrine  of,  18 
Epihyal,  376 

Epiotic  center  of  ossification,  366 
Epithelium,  germinal,  27,  29, 221 
Epitrichium,  246 
Eponychium.  248 
Epoophoron,  231 
Ethmoid  bone,  ossification  of,  369 

cribriform  plate  of,  362 
Ethmoidal  sinus,  development  of,  337 
Eustachian  tube,  development  of,  331, 
332 
formation  of,  177 

valve,  144 
Evertebral  region  of  primitive  skull, 

361 
Exoccipitals,  364 
Exoskeleton,  347 

Exstrophy  of  bladder,  cause  of,  74 
External  auditory  meatus,  formation 
of,  332 

ear,  development  of,  331,  333 

fertilization,  38 

genitals,  female,  236,  243 
male,  237,  244 

organs  of  generation,  234 
Eye,  development  of,  122,  302 
Eyelashes,  development  of,  319 
Eyelid,  third,  319 
Eyelids,  development  of,  318 

primitive,  122 


Face,  development  of,  106,  118 
Facial  ganglion,  297 
Falciform  ligament  of  liver,  forma- 
tion of,  193 

lobe,  285,  289 
Fallopian  tubes,  development  of,  230 
False  amnion,  73 
Falx  cerebri,  278 
Fecal  fistula,  congenital,  190 
Female  external  genitals,  236, 243 

internal  genital  organs,  226 

pronucleus,  32 

sexual  system,  243 
Fertilization,  38 

artificial,  40 

external,  38 

internal,  38 
Fetal  arterial  system,  149 

membranes  at  birth,  94 

vascular  system,  final  stage  of,  165 

venous  system,  153 
Fetus,  length  of,  at  term,  111 

stage  of,  20,  107 

weight  of,  at  term,  111 
Fiber-tracts  of  cord,  development  of, 
261 
myelination  of,  388,  389 
Fibrillse  of  muscle,  formation  of,  342 
Fibrous  tunic  of  eye,  development  of, 

315 
Fifth    brain-vesicle,    metamorphosis 
of,  264 

month,    development    during,   110, 
388 

pair    cranial   nerves,  development 
of,  299 

ventricle,  288 

week,  development  during,  385 
Fimbria,  285 

Fingers,  development  of,  381 
First   pair    ci'anial    nerves,   develop- 
ment of,  299 

week,  development  during,  393 
Fissure,  arcuate,  282,  283 

calcarine,  279,  284 

calloso-marginal,  285 

choroid,  282,  283 

choroidal,  306 

collateral,  279,  284 

dentate,  279,  283 

great  transverse,  280,  284 

hippocampal,  283 

of  choroid  plexus,  283 

of  Rolando,  284 

of  Sylvius,  279,  280 

parieto-occipital,  284 
Fissures,  cerebral,    development    of, 
278,  279 

median  of  cord,  261 
Fistula,  congenital  fecal,  190 

umbilical  urinary,  233 
Flexure,  cephalic,  101,  264 

nuchal,  264 

pontal,  264 
Floor-plate,  257,  258 


396 


INDEX. 


Fold,  pleuropericardial,  160 
Folds,  medullary,  64 
Follicle,  Graafian,  27 

of  tooth,  129 
Foramen  caecum,  133,  208 

commune  auterius,  282 

of  Monro,  277,  282 

of  Winslow,  203 

ovale,  143 
Fore-brain,  262,  278 

secondary,  263 

vesicle,  65 

metamorphosis  of,  278 
Foregut,  73 
Formative  yolk,  24 
Fornix,  foi-mation  of,  285,  286 
Fossa  of  Sylvius,  280 

oral,  175 
Fourth  month,  development  during, 
109,  388 

pair  cranial    nerves,    development 
of,  299 

ventricle,  267 

development  of,  265,  270 

week,  development  during,  384 
Fretum  Halleri,  142 
Frontal  bone,  ossification  of,  370 

lobe,  282 

sinuses,  development  of,  337 
Funiculus  solitarius,  266 

Gall-bladder,  development  of,  192 
Ganglia,  cephalic,  296 

spinal,  293 
Gangliated  cord  of  the  sympathetic, 

301 
Ganglion,  acoustic,  297 
acusticofacial,  297 
cephalic,  fourth,  297 

third,  297 
ciliary,  296 
cochlear,  297 
facial,  297 
Gasserian,  396 
ophthalmic,  296 
spirale,  326 
trigeminal,  297 
vestibular,  326 
Ganglion-cell  layer,  development  of, 

309 
Gartner,  duct  of,  231 
Gasserian  ganglion,  296 
Gastral  mesoderm,  55 
Gastrohepatic  omentum,  192,  202 

formatio7i  of,  188 
Gastrosplenic  omentum,  195 
Gastrula,  47 
mammalian,  48 
stage,  47 
Generative  organs,  external,  develop- 
ment of,  234 
internal,  development  of,  220 
Genital  cord,  220 
eminence,  235 
in  male,  238 


Genital  folds,  235 
in  femalp,  236 
in  male,  239 

gland,  indilferent,  242 

groove,  235 

ridge,  220,  233 
in  female,  237 

ridges,  29 
Genito-urinary  system,  development 

of,  212,  383-390 
Germ-cells,  221 
Germ-disk,  25 
Germ-layers,  47 

derivatives  of,  58 
Germinal  epithelium,  27,  29,  221 

spot,  23,  25 

vesicle,  23,  25 
Germinative  disk,  26 
Giraldes,  organ  of,  224 
Glands  of   alimentary  tract,  forma- 
tion of,  189 

of  Bartholin,  237 

of  Brunner,  development  of,  189 

of  Cowper,  development  of,  239 

of  intestine,  development  of,  189 

of    Lieberkiihn,    development    of, 
189 

of  Moll,  250 

of  stomach,  development  of,  189 
Glandular  area,  251 

hypospadias,  239 
Glans  clitoridis,  formation  of,  235 

penis,  formation  of,  235,  238 
Glaserian  fissure,  367,  373 
Globular  processes,  107,  120,  334 
Glomerulus  of  kidney,  213,  216,  217 
Goll,  tract  of,  myelination  of,  388 
Graafian  follicle,  27 

development  of,  228 
formation  of  new,  229 
Gray  matter  of  brain,  formation  of, 
279 
of  medulla,  development  of,  206 
Great  omentum,  formation   of,   187, 

201 
Groove,  dental,  126 

lacrimal,  108,  120 

medullary,  63 

naso-optic,  320 

primitive,  52 

pulmonary,  205 

transverse  crescentic,  372 
Gubernaculum  testis,  225 
Gum,  development  of,  124 
Gut,  postanal,  179 
Gut-tract,  72,  73,  169,  171 
Gyrus  fornicatus,  291 

uncinatus,  291 

Hair,  development  of,  248 
Hair-bulb,  248 

development  of,  249 
Hair-follicle,  248 

development  of,  249,  250 
Hair-germs,  249 


INDEX. 


397 


Hard  palate,  development  of,  371 
Hare-lip,  122,  371 
Hassal,  corpuscles  of,  210,  211 
Head,   muscles    of,   development    of, 
343 
of  epididymis,  223 
of  spermatozoon,  20 
Head-fold,  72 

of  amnion,  72,  75 
Head-gut,  171 
Head-kidney,  212 
Head-process  of  primitive  streak,  54, 

62 
Head-segments,  340 
Head-skeleton,  development  of,  358 
Heart,  development  of,  138 

metamorphosis  of  single  into  double, 

142 
valves,  development  of,  144 
Helix,  formation  of,  333 
Hemal  arch,  formation  of,  352 
Hen's  egg,  description  of,  25 
Hensen's  node,  54 
Hepatic  cylinders,  192 

vein,  development  of,  164 
Hermaphroditism,  239,  244 
Hernia,  congenital,  226 

umbilical,  188 
Highmore,   antrum  of,   development 

of,  336 
Hilum  folliculi,  29 
Hind-brain,  262,  268 
secondary,  263 
vesicle,  65,  268 
Hindgut,  73,  171 
Hippocampal  fissure,  283 
Hippocampus  major,  283 

minor,  284 
His,  canal  of,  133,  208 
Holoblastic  ova,  43 
Homogeneous  twins,  origin  of,  51 
Homologies  of  the  sexual  system,  240 
Hyaloid  artery,  formation  of,  314 
canal,  315 

membrane,  formation  of,  315 
Hydatid  of  Morgagni,  224 
sessile,  224 
stalked,  224 
unstalked,  224 
Hydramnios,  77 
Hymen,  formation  of,  237 
Hyoglossus,  origin  of,  345 
Hyoid  arch,  anterior,  363 
posterior,  363 
arches,  104 
bar,  363 

bone,  development  of,  363,  375 
Hyoidean  apparatus,  375 
Hyomandibular  cleft,  104 
Hypoblast,  47 
Hypochordal  brace,  351 
Hypophysis,  276 

formation  of,  123 
Hypospadias,  239 
glandular,  239 


Iliac  segment  of  pelvic  girdle,  378 
vein,  left  common,  development  of, 
156 
Imperforate  anus,  180 
Impressions,  maternal,  109 
Incus,  development  of,  362,  373 
Indifferent  genital  gland,  242 

sexual  gland,  221 
Inferior  medullary  velum,  268 

peduncles  of  brain,  266 
Infundibula  of  lungs,  development  of, 

207 
Infundibulum  of  brain,  272,  276 
Inguinal  ligament,  225 

in  female,  231 
Inner  cell-mass,  45 
Innominate  artery,  development  of, 

152 
Inter-brain,  263,  272 

vesicle,  metamorphosis  of,  272 
Intermaxillary  bones,   formation  of, 

124,  371 
Intermedial  cell-mass,  69,  341 
Internal  ear,  development  of,  321 
fertilization,  38 

lateral  ligament  of  lower  jaw,  374 
limiting  membrane  of  spinal  cord, 
259 
Interpallial  fissure,  278 
Intervertebral  disks,  351,  353 

ligament,  development  of,  352,  353 
Intervillous  spaces,  87,  92 
Intestinal  canal,  formation  of,  71 
glands,  development  of,  189 
mesentery,  198 
mucosa,  formation  of,  172 
villi,  formation  of,  189 
portals,  73,  169 
Intestine,  small,  development  of,  185, 

188 
Intestiuo-body  cavity,  47 
Intumescentia  ganglioformis,  326 
Involuntary  muscle,  development  of, 

346 
Iris,  coloboma  of,  318 
development  of,  317 
Ischiatic  rod,  378 
Island  of  Reil,  281 

Jacobson's    organ,   development  of, 

337 
Jaw,  upper,  development  of,  122 
Jaw-arch,  104 
Jelly  of  Wharton,  93 
Joint-cavities,  development  of,  117 
Jugular  vein,  primitive,  148,  154 
transverse,  156 

Kidney,  development  of,  212 

Labia  majora,  237 

minora,  formation  of,  237 
Labyrinth,  bony,  development  of,  326 

membranous,  development  of,  321 
Lacrimal  bones,  ossification  of,  370 


398 


INDEX. 


Lacrimal  canaliculi,  320 

caruncle,  319 

duct,  development  of,  320 

gland,  development  of,  319 

groove,  108,  120 

sac,  development  of,  321 
Lamina  cinerea,  272,  275 

quadrigemina,  271 

spiralis,  bony,  development  of,  330 

terminalis,  2b5 
Langhans'  layer,  87 
Lanugo,  110,  250 
Larynx,  development  of,  207 
Latebra,  27 
Lateral  cartilage  of  nose,  369 

folds  of  amnion,  72 

frontal  processes,  107,  120,  122 
in  formation  of  nose,  134 

ligaments  of  liver,  193 

nasal  process,  320,  334 

plate  of  mesoderm,  57 

plate  of  somite,  55 

ventricle,  development  of,  279 
Length  of  fetus  at  term,  111 
Lens,    crystalline,    development    of, 

312 
Lens-area,  304 

Lens-capsule,  development  of,  313 
Lens-pit,  312 

Lens-vesicle,  99,  122,  304,  312 
Lenticular  zoue  of  optic  cup,  309 
Lesser  omentum,  202 
formation  of,  188 
Levator  palati,  origin  of,  345 
Lids,  union  of  edges  of,  319 
Lieberkiihn,  glands  of,  189 
Ligament  of  ovary,  232 
Ligaments  of  liver,  formation  of,  192 
Ligamenta  intermuscularia,  350 

subflava,  353 
Ligulse,  267 
Limb-buds,  108,  380 
Limbic  lobe,  285,  289 
Limb-muscles,  development  of,  345 
Limbs,  bones  of,  development,  379 

development  of,  380,  383-390 

position  of,  381 
Limiting  membrane,  inner,  formation 
of,  307 
outer,  formation  of,  307 
Lip,  upper,  development  of,  124 
Liquor  amnii,  76,  77 
function  of,  77 

folliculi,  29,  228 

of  Morgagni,  313 
Liver,  development  of,  190 

first  rudiment  of,  181 

ligaments  of,  formation  of,  192 
Liver-ridge,  159,  191 
Lobes  of  liver,  191 
Lobule  of  ear,  development  of,  333 
Longitudinal  fiber-tracts  of  medulla, 
266 

fissure  of  brain,  278 
Lower  jaw,  ossification  of,  372 


Lumbar  rib,  357 

vertebrae,  ossification  of,  355 
Lungs,  development  of,  205 
Lymph,  formation  of,  115 
Lymph-clefts,  development  of,  117 
Lymph-sacs,  development  of,  116 
Lymph-spaces,  development  of,  116 
Lymphatic  system,  development  of, 
116 

vessels,  development  of,  117 
Lymphoid  follicles  of  tonsil,  178 

tissue,  development  of,  117 

Macula  lutea,  formation  of,  309 
Maculse  acusticse,  development  of,  325 
Malar  bone,  ossification  of,  370 
Male  external  genitals,  237,  244 

internal  genital  organs,  222 

pronucleus,  39 

sexual  system,  222,  242 
Malleus,  development  of,  362,  373 
Malpighian    corpuscle,    development 
of,  195,  217 
primitive,  216 
Mammalia  deciduata,  89 

indeciduata,  89 
Mammals,  blastula  of,  45 
Mammary  gland,  development  of,  251 
Mandible,  ossification  of,  372 
Mandibular  arch,  104,  123,  360 
Mantle  layer,  260 
Marginal  sinus,  92 

zone  of  optic  cup,  310 
Marshall,  vestigial  fold  of,  156 
Maternal  impressions,  109 
Maturation  of  ovum,  30 

theories  of,  32 
Maxilla,  superior,  ossification  of,  371 
Maxillary  arch,  104 

process,  123,  360 
Meatus,  external  auditory,  332 

urinarius,  male,  239 
Meckel's  cartilage,  104,  362,  372 

diverticulum,  formation  of,  190 

ganglion,  298 
Meconium,  111 
Median  fissures  of  cord,  261 

lobe  of  cerebellum,  268 
Medulla  oblongata,   development  of, 

265 
Medullary  canal,  62 

cords.  229 

folds,  64,  255 

furrow,  63 

groove,  63 

plate,  62,  255 

tube,  255 

velum,  anterior,  269 
inferior,  268,  270 
Meibomian  glands,   development  of, 

319 
Membrana  adamantina,  127 

basilaris  of  cochlea,  formation  of, 
330 

eboris.  129 


INDEX. 


3i.9 


Membrana  granulosa,  29 
formation  of,  228 

prEeformativa,  129 
Membrane,  anal,  176 

closing,  102,  106,  177 

nuclear,  25 

of  Nasmytb,  128 

of  Eeissner,  330 

pharyngeal,  106,  119, 171,  175 

vitelline,  23,  24 

tympanic,  177,  332 
Membranes,  caducous,  85 

deciduous,  85 
Membranous  bones,  359 

cranium,  359 

labyrinth,  development  of,  321 

ribs,  356 

stage  of  skeleton,  348 
of  trunk,  349 
Menopause,  35 
Menstrual  cycle,  35 
Menstruation,  35 

relation  of,  to  ovulation  and  con- 
ception, 37 
Meroblastic  ova,  44 
Mesencephalon,  270 
Mesenchymal  cells,  58 

muscle,  34b 
Mesenchyme,  58 
Mesenteries,  173 
Mesentery,  intestinal,  198 

ventral,  187 

development  of,  202 
Mesoblast,  54 

Mesoblastic  somites,  57,  67 
Mesocardium  anterius,  139,  158 

poster  ins,  139,  158 
Mesocolon,  ascending,  production  of, 
186 

formation  of,  186 
Mesoderm,  54 

derivatives  of,  60 

gastral,  55 

paraxial,  57 

peristomal,  55 

somatic,  58 

splanchnic,  58 

structures  developed  from,  113  et 
seq. 
Mesogastrium,  187,  198 
Mesonephric  duct,  214 
Mesonephros,  213,  241 
Mesorchium,  225 
Mesothelium,  58,  115 
Mesovarium,  225 
Metacarpal    bones,    development    of, 

379 
Metamorphosis  of  single  into  double 

heart,  142 
Metanephros,  217,  241 
Metatarsal    bones,    development    of, 

379 
Metencephalon,  264 
Metopic  suture,  370 
Metopism,  370 


Micropyle,  23,  39 
Mid-brain,  262,  270 

prominence  of,  264 

vesicle,  65,  270 
Mid-gut,  171 
Middle  ear,  development  of,  177,  331 

piece  of  spermatozoon,  20 

plate,  69,  212,  341 

sacral  artery,  development  of,  150 

tunic  of  eye,  development  of,  315 
Modiolus  of  cochlea,  development  of, 

329 
Moll,  glands  of,  250 
Monorchism,  226 
Monro,  foramen  of,  277,  282 
Mons  veneris,  formation  of,  237 
Morgagni,  hydatid  of,  224 

liquor  of,  313 
Morula,  41 
Mother-cells,  22 
Motor  nerve-fibers,  development  of, 

295 
Mouth,  development  of,  122,  175 
Mucous  tissue,  formation'  of,  114 
Mulberry-mass,  41 
Miiller,  duct  of,  220,  224,  230,  242 
Miiller's  fibers,  307 

Muscle,  involuntary,  development  of, 
346 

voluntary,  development  of,  339 
Muscle-plate,  69,  341 

metamorphosis  of,  342 
Muscles,    branchial,  development  of, 
344 

of  extremities,  development  of,  345 

of  trunk,  development  of,  339 
Muscular  coat  of  intestines,   forma- 
tion of,  188 

system,  development  of,  339, 383-390 
Musculi  papillares,  147 

pectinati,  140 
Myoccel,  341 
Myotome,  69,  341 

Nail-bed,  248 
Nail-plate,  247 
Nails,  development  of,  247 

of  toes,  248 
Nail-welt,  248 
Nares,  anterior,  formation  of,  134 

development  of,  336 
Nasal  areas,  133,  334 

bones,  ossification  of,  370 
capsule,  362 

cavities,  development  of,  336 
pits,  107,  120,  133,  334 
process,  120,  334 
lateral,  320,  334 
Nasmyth,  membrane  of,  128 
Nasofrontal  process,  104,  107,  120, 122, 
334,  360 
in  development  of  nose,  133 
Naso-optic  furrow,  120,  122 
in  formation  of  nose,  134 
groove,  320 


400 


INDEX. 


Nephrotome,  69,  214,  241,  341 
Nerve-cells,  formation  of,  258 

of  cord,  formation  of,  260 
Nerve-corpuscles  of  neurilemma,  295 
Nerve-fiber,  envelopes   of,  formation 
of,  295 

layer,  development  of,  309 
Nerve-fibers,  cranial,  development  of, 
296 

motor,  development  of,  295 

sensory,  development  of,  293 
Nerve-trunk,  spinal,  development  of, 

295 
Nervous  system,  development  of,  254, 
383-390 
peripheral,  development  of,  292 
sympathetic,  development  of,  300 
Neural  arch  of  vertebra,  formation  of, 
352 

canal,  62,  255,  257 

crest,  segmentation  of,  294 

crests,  294 

tube,  255 
Neurenteric  canal,  66,  257 
Neurilemma,  formation  of,  295 
Neurit,  254,  260 
Neuroblasts,  258,  260 
Neuro-epithelium  of  retina,  develop- 
ment of,  309 
Neuroglia,  258,  259 

layer,  260 
•Neurons,  254 

Nictitating  membrane,  319 
Ninth  month,  development   during, 
111,  390 

pair  cranial  nerves,  development  of, 
300 

week,  development  during,  387 
Nipple,  development  of,  252 
Node,  Hensen's,  54 
Nose,  development  of,  133,  334 
Notochord,  65 

Notochordal  stage  of  skeleton,  348 
Nuchal  flexure,  264 
Nuck,  canal  of,  232 
Nuclear  juice,  25 

layer,  of  retina,  outer,  308 

membrane,  25 

spindle,  41 
Nucleus  amygdalae,  279 

cleavage-,  39 

of  ovum,  25 

segmentation-,  39 
Nutritive  yolk,  24 
Nymphae,  formation  of,  237 

Obex,  267 

Occipital  bone,  ossification  of,  364 

lobe,  282 
Odontoblasts,  129 
Odontoid    process,    development    of, 

355 
Olfactory  bulb,  290 

epithelium,  334 

lobe,  290 


Olfactory  nerve-fibers,  338 

plates,  120,  133,  334 

tract,  290 
Omental  bursa,  187,  200 
Omentum,  gastrohepatic,  192,  202 
formation  of,  188 

gastrosplenic,  195 

great,  formation  of,  187,  201 

lesser,  202 
formation  of,  188 

phrenicosplenic,  195 
Omphalomesenteric  veins,  136 
Ontogeny,  17 
Oogenesis,  27 
Ophthalmic  ganglion,  296 
Opisthotic  center  of  ossification,  366 
Optic  cup,  304 

secondary,  306 

lobes,  formation  of,  271 

nerve,  development  of,  311 

thalami,  272 

vesicle,  263,  303 
Ora  serrata,  307 
Oral  cavity,  development  of,  175 

fossa,  175 

pit,  97,  106,  119,  123,  175 

plate,  118,  122,  175 
Organ  of  Corti,  325 

of  Giraldes,  224 

of  Jacobson,  development  of,  337 

of  Eosenmiiller,  231 
Osseous  cranium,  363 

stage  of  trunk  skeleton,  354 

tissue,  formation  of,  115 
Ossicles  of  ear,  development  of,  332 
Ossification  of  ribs,  357 

of  skull,  363 

of  sternum,  357 

of  vertebrae,  354 
Otic  vesicle,  98,  322 
Otocyst,  322 
Outer  cell-mass,  45 
Ova,  alecithal,  24 

centrolecithal,  25 

classification  of,  24 

formation  of,  27 

holoblastic,  43 

meroblastic,  44 

primitive,  29,  222,  227 

telolecithal,  24 
Ovaries,  change  of  position  of,  231 
Ovary,  development  of,  226 
Oviducts,  development  of,  230 
Ovists,  18 
Ovulation,  32 

relation  of,  to  menstruation,  34 
Ovum,  22,  228 

embedding  of,  85,  86 

maturation  of,  30 

ripening  of,  30 

scgnientation  of,  41 

stage  of,  19,  96 

Palate  bone,  ossification  of,  370 
formation  of,  124 


INDEX. 


401 


Palate  process,  development  of,  371 
Palate-shelves,  336 
Palatoglossus,  origin  of,  345 
Palatopharyugeus,  origin  of,  345 
Palpebral  fasciae,  319 

fissure,  319 
Pancreas,  development  of,  194 

first  rudiment  of,  182 
Pancreatic  duct,  development  of,  194 
Pander,  blood-islands  of,  135 
Pander's  nucleus,  26 
Panniculus  adiposus,  246 
Papillae  of  tongue,  formation  of,  133 
Parablast,  58 

Parachordal  cartilages,  361 
Paradidymis,  224 
Paraxial  mesoderm,  57 
Parietal  bones,  ossification  of,  370 

elevation,  264 

eye,  275 

foramen,  274 

layer  of  pleura,  161 

lobe,  282 

zone,  68 
Parieto-occipital  fissure,  284 
Paroophoron,  231 
Parovarium,  231 
Pars  ciliaris  retinae,  310 

iutermedialis,  237 

iridica  retiuee,  310 

optica  retinae,  309 
Parthenogeuetic  eggs,  32 
Patulous  foramen  ovale,  143 
Pectoral  girdle,  development  of,  377 
Pelvic  girdle,  378 

Pelvis  of  kidney,  formation  of,  218 
Penis,  development  of,  235 
Perforated  lamina,  anterior,  291 

space,  posterior,  270,  271 
Pericardial  cavity,  159 
Pericardium,  development  of,  158 
Perilymph,  327,  331 
Perilymphatic  space,  327 
Perineal  body,  180 
Perineum,  formation  of,  180 
Perionyx,  248 
Periotic  bone,  366 
Peripheral  cleavage,  44 

nervous  system,  292 
Peristomal  mesoderm,  55 
Peritoneal  cavity,  198 
Peritoneum,  development  of,  195 

visceral  layer  of,  172 
Perivitelline  space,  23 
Permanent  kidney,  217 

teeth,  development  of,  129 
eruption  of,  131 
Petromastoid  bone,  366 
Petrotympanic  fissure,  373 
Pfliiger's  egg-tubes,  227 
Phalanges,  development  of,  379 
Pharyngeal  bursa,  124 

constrictors,  origin  of,  345 
membrane,  106,  119,  171,  175 
in  formation  of  mouth,  123 
26 


Pharvngeal  pouches,  102,  171,  176 
Pharynx,  176 

Phrenicosplenic  omentum,  195 
Phylogeny,  17 
Pial  processes,  259 
Pigment-layer  of  retina,  307 
Pillars  of  Uskow,  161 
Pineal  body.  272 

or  gland,  273,  274 

eye,  275 
Pit,  auditory,  322 

oral,  97,  106 
Pits,  nasal,  334 
Pituitary  body,  276 

formation  of,  123 
Placenta,  88 

at  term,  91 

discoidea,  89 

praevia,  92 

zonaria,  89 
Placental  sinuses,  90 

spaces,  92 

system  of  blood-vessels,  149 
Planes  of  cleavage,  42 
Plantar  horn,  247 
Plate,  chordal,  66 

medullary,  62 

vertebral,  57 
Pleura,  parietal  layer  of,  161 

visceral  layer  of,  161 
Pleurae,  development  of,  158, 159 
Pleural  sacs,  formation  of,  158,  159 
Pleuropericardial  fold,  160 
Pleuroperitoneal  cavity,  58,  197 
Plica  semilunaris,  319 
Pocket  of  Eathke,  277 
Polar  bodies,  31 

striation,  41 
Polarity  of  egg,  25 
Pole-corpuscles,  30 
Polyphyodont,  125 
Polyspermia,  39 
Pontal  flexure,  264 
Pons,  formation  of,  268 
Portal  circulation,  154,  161 

vein,  development  of,  164 

venous  system,  154,  161 
Postanal  gut,  179 
Posterior  chamber  of  eye,  318 

nares.  development  of,  336 
Post-limbic  sulcus,  285 
Preformation  theory,  18 
Prehepaticus,  159,  191 
Prehyoid  gland,  209 
Premaxilla,  371 
Prepuce,  I'crmation  of,  238' 
Presphenoid,  368 
Primary  egg-tubes,  29 
Primitive  aorta,  137,  150 

chorion,  82 

enamel-germ,  126 

eyelids,  122 

groove,  52 

heart-valves,  144 


402 


INDEX. 


Primitive  jugular  veins,  148,  154 

Malpighiau  corpuscle,  216 

nails,  247 

ova,  29,  222,  227 

red  blood-corpuscles,  136 

segment  plate,  57 

segments,  57,  67 

sexual  cells,  221 

streak,  51 

vertebral  bow,  351 
Primordial  bones,  359 
Proamnion,  56 
Process,  lateral  frontal,  107, 120,  122 

nasal,  120,  334 

nasofrontal,  104,  107,  120,  122,  334 
in  formation  of  nose,  133 
Processes,  dental,  129 

globular,  107,  120,  334 
nasal,  334 

nmxillary,  123 

of  vertebra,  development  of,  352 
Processus  vaginalis,  226 
Prochorion,  45,  82 
Proctodeum,  179 
Pronephric  duct,  212 
Pronephros,  212,  241 
Pronucleus,  female,  32 

male,  39 
Pro-otic  center  of  ossification,  366 
Prosencepbalon,  278 
Prostate  gland,  formation  of,  234 
Prostatic  urethra,  formation  of,  234 
Protoplasmic  processes,  260 
Protovertebra,  55 

Pterygoid    plate,    internal,   develop- 
ment of,  368 
Pubic  rod,  378 

Pulmonary   alveoli,  development  of, 
207" 

artery,  development  of,  144,  152 

diverticulum,  205 

groove,  205 
Pulp  of  spleen,  development  of,  195 

of  teeth,  125 
Pupil,  306 

congenital  atresia  of,  314 

development  of,  318 
Pyramidal    tracts,  anterior   develop- 
ment of,  266 
crossed,  of  cord,  myelination  of, 
389 

Eamus  communicans,  301 
Eatlike's  pocket,  124,  277 
Eauber's  layer,  45,  49 
Eeceptive  prominence,  39 
Eecessus  labyrinthi,  322 

vestibuli,  322 
Rectum,  180 

Recurrent  laryngeal  nerves,  152 
Red  blood-corpuscle,  primitive,  136 
Reichert's  cartilage,  104,  363,  375 
Reil,  island  of,  281 
Reissner,  membrane  of,  330 
Renal  vein,  left,  157 


Reproduction,  theories  of,  17 
Respiratory  system,  development  of, 

204,  383-3y0 
Restiform  bodies,  development  of,  266 
Rete  mucosum,  247 

testis,  formation  of,  223 
Retina,  development  of,  304 
Rhiuencepbalou,  290 
Rhomboidal  fossa,  267 
Rib,  cervical,  354,  357 

lumbar,  357 

thirteenth,  357 
Ribs,  development  of,  356 
Ridge,  genital,  220 

terminal,  50 
Ring  lobe,  formation  of,  280 
Ripening  of  ovum,  30 
Rod  and  cone-layer,  formation  of,  308 
Rod-visual  cells,  307 
Rolando,  fissure  of,  284 
Roof-plate,  257,  258 
Rotation  of  stomach,  186,  200 
Round  ligament  of  liver,  167 
formation  of,  193 
of  uterus,  225,  232 

Saccule,  development  of,  324 
Saccus  endolymphaticus,  323 
Sacral  vertebrae,  ossification  of,  355 
Sacrum,  formation  of,  355 
Salivary  glands,  development  of,  131 
Sauropsida,  blastula  of,  46 
Scala  media  of  cochlea,  development 
of.  323 

tympaui,  development  of,  330,  331 

vestibuli,  development  of,  330,  331 
Scapula,  develoi>ment  of,  377 
Schwann,  white  substance  of,  295 

deposit  of,  upon  fibres  of  tract  of 
cord,  388,  389 
Sclerotome,  69,341,  350 
Scrotum,  development  of,  239 
Sebaceous  glands,  development  of,  251 
Second  mouth,  development  during, 
107 

pair  cranial  nerves,  development  of, 
299 

week,  development  during,  383 
Secondary  hair,  250 

optic  cup,  306 
Segmental  duct,  213 
Segmentation  of  body  of  embryo,  69 

of  ovum,  41 
Segmentation-cavity,  45 
Segmentation-nucleus,  39 
Semicircular  canals,  bony,  327 

development  of,  323,  324 
Semilunar  valves,  development  of,  147 
Seminal  ampullse,  223 

vesicle,  formation  of,  224 
Seminiferous  tubules,    formation   of, 

223 
Sense  organs,    development   of,   302, 

383-390 
Sensory  epithelium  of  retina,  307 


INDEX. 


403 


Sensory  nerve-fibers,  development  of, 

293 
Septa  placentse,  92 
Septal  cartilage  of  nose,  369 
Septum  lucidum,  formation  of,  288 

transversum,  148,  159 
Serosa,  73 
Serous  membranes,  development  of, 

.  115 
Sertoli's,  columns,  21,  223 
Sessile  hydatid,  224 
Seventh  month,  development  during, 
110,  389 
pair    cranial   nerves,    development 

of,  299 
week,  development  of,  during,  386 
Sexual  cells,  29 
primitive,  221 
cords,  29,  222 
female,  227 
gland,  indifferent,  221 
segment  of  Wolfl&an  body,  219 
system,  female,  226,  243 
homologies  of,  240 
indifiFerent  tvpe,  220 
male,  222,  242 
Shell  of  hen's  egg,  27 
Shell-membrane,  27 
Shoulder  girdle,  development  of,  377 
Sinus,  annular,  163 
pocularis,  224,  234 
prfficervicalis,  105 
reuniens,  144 
terminalis,  136 
urogenital,  173,  233 
venosus,  144,  153 
Sixth    month,    development    during, 
110,  389 
pair  cranial  nerves,  development  of, 

299 
week,  development  during,  108,  385 
Skeletogenous  sheath  of  chorda  dor- 
sal! s,  350 
tissues,  69 
Skeleton,  appendicular,  348 

development  of,  376,  333-390 
axial,  348 

development  of,  347 
of  head,  development,  of,  358 
of    trunk,   cartilaginous    stage    of, 
351 
chordal  stage  of,  348 
development  of,  348 
membranous  stage  of,  349 
visceral,  358 
Skin,  appendages  of,  247 

development  of,  245,  383-390 
Small  intestine,  development  of,  188 
Smegma  embryonum,  247 
Somatic  mesoderm,  58 
Somatopleure,  58,  169 
Somites,  .55,  67 

mesoblastic,  57,  67 
Space,  peri  vitelline,  23 
Spaces,  intervillous,  87,  92 


Spermatic  cord,  226 

veins,  157 
Spermatids,  22 
Spermatoblasts,  22 
Spermatogenesis,  21 
Spermatogenic  cells,  21 
Spermatozoon,  20 

power  of  locomotion  of,  21 

vitality  of,  21 
Sphenoid  bone,  ossification  of,  367 
Sphenoidal  sinus,  development  of,  337 
Sphenopalatine  ganglion,  296 
Spinal  cord,  development  of,  257 
Spinous  process  of  vertebra,  develop- 
ment of,  354 
Splanchnic  mesoderm,  58 
Splanchnopleure,  58,  169 
Spleen,  development  of,  194 
Spongioblasts,  258,  259 
Spot,  germinal,  25 
Squamozygomatic  bone,  365 
Stage  of  embrvo,  19,  96 

of  fetus,  20, 107 

of  ovum,  19,  96 

of  quiescence  of  menstrual  cycle, 
36 

of  repair  of  menstrual  cycle,  36 
Stalked  hydatid,  224 
Stapes,  development  of,  363 
Stem-zone,  67 
Sternum,  cleft,  357 

development  of,  357 
Stigma,  29 

Stilling,  canal  of,  315 
Stomach,  development  of,  186 

first  rudiment  of,  181 

glands  of,  development  of,  189 

rotation  of,  186,  200 
Stomodgeura,  119,  175 
Stratum  Malpighii,  247 
Streak,  primitive,  51 
Striated  muscles,  development  of,  .339 
Stroma-laver  of  choroid,  development 

of,  316 
Styloglossus,  origin  of,  345 
Stylohyal,  376 

cartilage.  367 
Stylohyoid  ligament,  363 
Styloid  process  of  hyoid,  363 

temporal,  development  of,  367 
Stylopharyngeus,  origin  of,  345 
Subclavian  artery,  left,  development 
of,  152 
right,  development  of,  151 
Submaxillary  ganglia,  296 
Submucosa  of  intestines,  formation  of, 

188 
Sulcus  interventricularis,  143 

of  corpus  callosum,  283 
Superior  maxilla,  ossification  of,  371 
Suprahyoid  gland,  209 
Supra-occipital  bone.  364 
Sui)rapericardial  borlies.  209 
Suprarenal    bodies,   development   of, 
218,  242 


404 


INDEX. 


Suspensory  ligameut  of  liver,  forma- 
tion of,  193 
Susteutacular    cells  of   seminiferous 

tubule,  21 
Suture,  amniotic,  75 
Sweat-glands,  development  of,  250 
Sylvius,  aqueduct  of,  271 

fissure  of,  279,  280 

fossa  of,  280 
Sympathetic  nervous  system,  300 
Syncytium,  83,  87 
Synovial  sacs,  development  of,  115 

Tail  of  spermatozoon,  20 

Tail-fold,  72 

Tarsal  ligaments,  319 

plates,  319 
Tarsus,  development  of  bones  of,  379 
Teeth,  development  of,  125 

permanent,  development  of,  129 
eruption  of,  131 

temporary,  development  of,  125 
eruption  of,  130 
Tela  choroidea,  273 
Telolecithal  ova,  24 
Temporal  bone,  ossification  of,  364 

lobe,  formation  of,  280 
Temporary  teeth,  development  of,  125 

eruption  of,  130 
Tempoiomasillary  articulation,  374 
Tendon,  development  of,  114 
Tendon-sheaths,  development  of,  117 
Tenth  pair  cranial  nerves,  develop- 
ment of,  300 
Terminal  ridge,  50 
Testicle,  development  of,  222 

descent  of,  225 
Thalamencephalon,  272 
Thebesius,  valve  of,  144 
Theca  folliculi,  27 
Thecal  sacs,  development  of,  115 
Theory  of  evolution,  17 

of  unfolding,  17 
Third  eyelid,  319 

month,    development  during,   109, 
387 

pair   cranial    nerves,   development 
of,  299 

ventricle,  formation  of,  272 

week,  development  during,  384 
Thirteenth  rib,  357 
Thoracic  prolongations  of  abdominal 

cavity,  159 
Throat-pockets,  102,  171,  176 
Thymus  body,  177,  210 
Thyroglossal  duct,  133,  208 
Thyroid  body,  accessory,  209 
development  of,  177,  207 

duct,  208 

foramen,  378 
Tissue  fungus,  87 
Toes,  development  of,  381 
Tongue,  development  of,  131,  177 
Tonsil,  develoi)ment  of,  177 
Tonsillar  pit,  178 


Trabeculse  cranii,  361 
Trachea,  development  of,  207 
Tragus,  formation  of,  333 
Transverse  colon,  formation  of,  186 

crescentic  groove,  72 

fissure  of  brain,  273 

processes  of  vertebrae,  354 
Trigeminal  ganglion,  296 
Trophoblast,  83 
True  chorion,  82 

Truncus  arteriosus,  102,  137,  140,  149 
Trunk,  skeleton  of,  development  of, 
348 
osseous  stage  of,  354 
Trunk-muscles,  development  of,  339 
Trunk-segments,  340 
Tuber  cinereum,  272,  276 
Tuberculum  impar,  132,  177 
Tubotympauic  sulcus,  331 
Tunica  albuginea  of  ovary,  227 

fibrosa,  28 

propria,  28 

vaginalis  testis,  226 

vasculosa,  27 
lentis,  313 
Turbinal  folds,  336 
Turbinate  bone,  inferior,  ossification 

of,  369 
Turbinated  bones,  development  of,  337 
Twelfth  pair  cranial  nerves,  develop- 
ment of,  300 
Twins,  origin  of,  51 
Tympanic  cavity,  formation  of,  177 

membrane,  177 

development  of,  332 

portion  of  temporal  bone,  develop- 
ment of,  367 
Tympanohyal,  376 

cartilage,  367 
Tympanum,  development  of,  331 

Umbilical  aperture,  78,  169 

arteries,  93,  149 

cord,  92 

hernia,  congenital,  189 

urinary  fistula,  233 

vein,  93,  149,  153 

vesicle,  72,  78,  169 
function  of,  80 
human,  80 

vessels,  93 
Uncinate  gyrus,  291 
Unstriated   muscle,  development  of, 

346 
Urachus,  82,  233 
Ureter.  218 

development  of,  212 
Urethra,  female,  234 

male,  formation  of,  239 

prostatic,  formation  of,  234 
Urinary  fistula,  umbilical,  233 
Uriniferous  tubules,  formation  of,  217 
Urogenital  aperture,  233 

sinus,  173,  179,  233 
Uskow,  pillars  of,  161 


INDEX. 


405 


Uterus  bicornis,  230 

development  of,  230 

double,  230 

masculinus,  224,  234 
Utricle,  development  of,  324 
Uveal  tract,  development  of,  316 
Uvula,  formation  of,  125 

Vagina,  development  of,  230 

median  septum  in,  230 
Valve,  coronary,  144 

Eustachian,  144 

of  Thebesius,  144 

of  Vieussens,  269 
Valves,  atrioventricular,  142 

auriculoventricular,  146 

of  heart,  development  of,  144 

semilunar,  development  of,  147 
Vas  deferens,  formation  of,  223 
Vasa  eiferentia,  223 

recta,  formation  of,  223 
Vascular  area,  79 

system,  development  of,   135,  383- 
390 
fetal,  final  stage  of,  165 

tunic  of  eye,  development  of,  315 
Vegetative  pole,  25 
Vein,  cardinal,  148 

hepatic,  164 

iliac,  left  common,  development  of, 
156 

portal,  development  of,  164 

renal,  left,  157 

umbilical,  93 
Veins,  allantoic,  81,  148 

cardinal,  154 

omphalomesenteric,  136 

primitive  jugular,  154 

spermatic,  157 

umbilical,  149,  153 

vitelline,  136,  153 
Velum  interpositum,  272,  273 
Vena  azygos  major,  157 
minor,  158 

cava  inferior,  155,  157 
superior,  154 
Vense  hepaticse  advehentes,  163 

revehentes,  163 
Venous  segment  of  heart,  141 

system  of  fetus,  153 
portal,  154 
Ventral  mesentery,  173,  187 

development  of,  202 
Ventricles,  separation  of,  143 
Vermiform     appendix,    development 
of,  186 

process  of  cerebellum,  268 
Vernix  caseosa,  78, 110,  247 
Vertebrae,  ossification  of,  354 
Vertebral  bow,  primitive,  351 

column,  development  of,  348-356 
membranous  primordial,  350 

plate,  57 


Vertebral  region  of  primitive  skull, 

361 
Vesicle,  germinal,  23,  25 

otic,  98,  322 

umbilical,  72,  78,  169 
Vesicles,  cerebral,  262,  263 

lens,  99 
Vestibular  ganglion,  326 

nerve,  297 
Vestibule  of  ear,  development  of,  327 

of  vagina,  237 

of  vulva,  234 
Vestigial  fold  of  Marshall,  156 
Vieussens,  valve  of,  269 
Villi  of  chorion,  83 

of  intestine,  formation  of,  189 
Visceral  arch,  first,  function  of,  104, 
119 

arches,  101 
metamorphosis  of,  104 
morphological  significance  of,  102 

clefts,  101  * 

layer  of  peritoneum,  172 
of  pleura,  161 

skeleton,  358 
Visceral-arch  vessels,  102,  137,  149 
Vitelline  arteries,  137 

circulation,  formation  of,  135 

duct,  72,  78,  169 

membrane,  23,  24 

veins,  136,  153 
Vitellus,  23,  24 

Vitreous  body,  development  of,  314 
Vocal  cords,  development  of,  207 
Voluntary  muscles,  development  of, 

339 
Vomer,  ossification  of,  370 

Weight  of  fetus  at  term,  111 

at  difi"erent  stages,  386-390 
Wharton,  jelly  of,  93 
White  commissures  of  cord,  260 
fibrous  tissue,  formation  of,  114 
matter  of  brain,  formation  of,  279 

of  cord,  development  of,  261 
of  hen's  egg,  27 

substance  of  Schwann,  development 
of,  295,  383,  389 
Winslow,  foramen  of,  203 
Witches'  milk,  253 
WolflBan  body,  213 
duct,  215 

in  female,  230 
Wolff's  doctrine  of  epigenesis,  18 
Wreath,  41 

Yolk  of  ovum,  23 
Yolk-sac,  72,  78,  169 

Zona  pellucida,  23,  29 

radiata,  29 
Zone,  parietal,  68 

stem-,  68 


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Anders* 
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Philadelphia. 
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A.  C.  Cowperthwedt,  M.  D., 

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"  I  consider  Dr.  Anders'  book  not  only  the  best  late  work  on  Medical  Practice,  but  by  far 
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in  everything.     I  consider  it  a  great  credit  to  both  the  author  and  the  pubHshers." 

George  Roe  Lockwood,  M.  D., 

Attending  Physician  to  the  Bellevue  Hospital,  New  York. 

"  I  have  read  several  of  the  important  chapters  carefully,  and  am  very  much  pleased  with 
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ALFRED   STENGEL.  M.D. 

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It  is  universally  acknowledged  that  the  Germans  lead  the  world  in  Internal  Medicine ;  and 
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SEE  NEXT  TWO  PAGES  FOR  LIST 


PRACTICE    OF  MEDICINE. 


AMERICAN  EDITION 

NOTHNAGEL'S  PRACTICE 

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Typhoid  and  Typhus  Fevers 

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though  it  is,  has  been  much  improved,  greatly  enlarged,  and  enhanced  in  value,  espe- 
cially to  American  readers.  .  .  .  The  monograph  on  typhoid  fever  is  the  best  exponent 
of  the  knowledge  that  we  have  in  regard  to  this  disease  that  is  to  be  had  in  any  lan- 
guage."— Jottrnal  of  the  American  Medical  Association. 

Smallpox  (including  Vaccination),  Varicella,  Cholera  Asiatica, 
Cholera  Nostras,  Erysipelas,  Erysipeloid,  Pertussis,  and 
Hay  Fever 

By  Dr.  H.  Lmmermann,  of  Basle  ;  Dr.  Th.  von  Jurgexsen,  of  Tubin- 
gen ;  Dr.  C.  Liebermeister,  of  Tiibingen  ;  Dr.  H.  Lenhartz,  of  Ham- 
burg ;  and  Dr.  G.  Sticker,  of  Giessen.  The  entire  volume  edited,  with 
additions,  by  Sir  J.  W.  Moore,  M.  D.,  F.  R.  C.  P.  I.,  Professor  of  Prac- 
tice, Royal  College  of  Surgeons,  Ireland.  Octavo,  682  pages,  illustrated. 
Cloth,  $5.00  net ;  Half  Morocco,  $6.00  net. 

"  Dr.  Immermann's  vindication  of  vaccination  in  the  prophylaxis  of  smallpox  will  be 
read  with  peculiar  interest  at  the  present  time,  since  it  is  probably  the  most  complete 
and  unassailable  indictment  of  the  propaganda  of  antivaccination  fanatics  which  has  ever 
been  published." — The  London  Lancet. 

Diphtheria,  Measles,  Scarlet  Fever,  and  Rotheln 

By  William  P.  Northrup,  M.  D.,  of  New  York,  and  Dr.  Th.  von 
Jurgensen,  of  Tubingen.  The  entire  volume  edited,  with  additions,  by 
William  P.  Northrup,  M.  D.,  Professor  of  Pediatrics,  University  and 
Bellevue  Hospital  ]\Iedical  College,  New  York.  Octavo,  672  pages,  illus- 
trated, including  24  full-page  plates,  3  in  colors.  Cloth,  $5.00  net  ;  Half 
Morocco,  $6.00  net. 

"  The  author  is  to  be  congratulated  on  the  exhaustive  and  practical  manner  in  which 
he  presents  the  subject.  .  .  .  The  articles  on  measles,  scarlet  fever,  and  German  measles 
are  exhaustive  treatises,  with  numerous  additions  by  the  American  editor." — Joui-nal 
of  the  American  Medical  Association. 

Diseases  of  the  Bronchi,  Diseases  of  the  Pleura,  and  In- 
flammations of    the  Lungs 

By  Dr.  F.  A.  Hoffmann,  of  Leipsic  ;  Dr.  O.  Rosenbach,  of  Berlin  ;  and 
Dr.  F.  Aufrecht,  of  Magdeburg.  The  entire  volume  edited,  with  additions, 
by  John  H.  Musser,  M.  D.,  Professor  of  Clinical  Medicine,  University  of 
Pennsylvania.  Octavo,  1029  pages,  illustrated,  including  7  full-page  colored 
lithographic  plates.     Cloth,  SS-oo  net  ;  Half  Morocco,  $6.00  net. 

"  These  monographs  in  the  original  hold  an  enviable  place  in  German  medical  literature, 
each  one  being  exhaustive,  complete,  authoritative,  and  written  by  men  specially  fitted  for 
the  work.  But  the  American  edition  is  not  only  a  reproduction  in  English,  it  is  all  of  this 
and  more  ;  for  the  American  editor  has  added  much  of  value  not  included  in  the  original,, 
and  he  has  brought  every  part  thoroughly  up  to  date." — Journal  of  the  American  Medical 
Association. 


SAUNDERS'    BOOKS   ON 


AMERICAN    EDITION 

NOTHNAGEL'S  PRACTICE 

VOLUMES  NOW  READY  AND  IN  PRESS 


Diseases  of  the  Pancreas,  Suprarenals,  and  Liver 

By  Dr.  L.  Oser,  of  Vienna  ;  Dr.  E.  Neusser,  of  Vienna,  and  Drs.  H. 
Quincke  and  G.  Hoppe-Sevler,  of  Kiel.  The  entire  volume  edited,  with 
additions,  by  Reginald  H.  Fitz,  A.  M.,  M.  D.,  Hersey  Professor  of  the 
Theory  and  Practice  of  Physic,  Harvard  University  ;  and  Frederick  A. 
Packard,  M.  D.,  Late  Physician  to  the  Pennsylvania  and  to  the  Children's 
Hospitals.  Octavo  of  918  pages,  illustrated.  Cloth,  $5.00  net;  Half  Mo- 
rocco, $6. 00  net. 

It  has  been  the  aim  of  the  authors  and  editor  of  this  work  to  describe  the  present  con- 
dition of  our  knowledge  on  the  subjects,  to  point  out  where  it  is  deficient,  and  to  stimu- 
late to  new  work.     The  work  will  be  found  practical  in  every  particular. 

Diseases  of  the  Stomach 

By  Dr.  F.  Riegel,  of  Giessen.  Edited,  with  additions,  by  Charles 
G.  Stockton,  M.  D.,  Professor  of  Medicine,  University  of  Buffalo.  Hand- 
some octavo  of  835  pages,  with  29  text-cuts  and  6  full-page  plates.  Cloth, 
15.00  net ;  Half  Morocco,  $6.00  net. 

This  work  is  a  complete  exposition  of  the  diseases  of  the  stomach.  Full  consideration 
is  given  to  the  hydrochloric  acid  question,  the  latest  views  being  incorporated  by  the 
editor.     Particular  attention  has  been  given  to  disturbances  of  motility  and  secretion. 

Diseases  of  the  Intestines  and  Peritoneum 

By  Dr.  Hermann  Nothnagel,  of  Vienna.  The  entire  volume  edited, 
with  additions,  by  Humphrey  D.  Rolleston,  M.  D.,  F.  R.  C.  P.,  Physi- 
cian to  and  Lecturer  on  Pathology  at  St.  George's  Hospital,  London.  Hand- 
some octavo  of  800  pages,  finely  illustrated. 

Influenza,  Dengue,  Malarial  Diseases 

By  Dr.  O.  Leichtenstern,  of  Cologne,  and  Dr.  J.  Mannaberg,  of 
Vienna.  The  entire  volume  edited,  with  additions,  by  Ronald  Ross, 
F.  R.  C.  S.,  Eng.,  D.  p.  H.,  F.  R.  S.,  Major,  Indian  Medical  Service,  retired  ; 
Walter  Myers  Lecturer,  Liverpool  School  of  Tropical  Medicine,  Liverpool. 
Handsome  octavo  of  700  pages,  with  7  full-page  lithographic  plates  in 
colors. 

Anemia,    Leukemia,    Pseudoleukemia,    Hemoglobinemia,    and 

Chlorosis 

By  Dr.  P.  Ehrlich,  of  Frankfort-on-the-Main  ;  Dr.  A.  Lazarus,  of 
Charlottenburg  ;  Dr.  Felix  Pinkus,  of  Berlin  ;  and  Dr.  K.  von  Noorden, 
of  Frankfort-on-the-Main.  The  entire  volume  edited,  with  additions,  by 
Alfred  Stengel,  M.  D.,  Professor  of  Clinical  Medicine,  University  of 
Pennsylvania.  Handsome  octavo  of  750  pages,  with  5  full-page  lithographs 
in  colors. 

Tuberculosis  and  Acute  General  Miliary  Tuberculosis 

By  Dr.  G.  C(jrni:t,  of  Berlin.  Edited,  with  additions,  by  Walter  B. 
James,  M.  D.,  Professor  of  the  Practice  of  Medicine,  Columbia  University, 
New  York.      Handsome  octavo  of  joo  pages. 

EACH  VOLUME  IS  COMPLETE  IN  ITSELF  AND  IS  SOLD  SEPARATELY 


THE  PRACTICE   OF  MEDICLXE 


Eichhorst's 
Practice  of  Medicine 

A  Text=Book  of  the  Practice  of  Medicine.  By  Dr.  Hermann  Eich- 
HORST,  Professor  of  Special  Pathology  and  Therapeutics  and  Director 
of  the  Medical  Clinic,  University  of  Zurich.  Translated  and  edited  by 
Augustus  A.  Eshner,  M.  D.,  Professor  of  Clinical  Medicine,  Phila- 
delphia Polyclinic.  Two  octavo  volumes  of  600  pages  each,  with  over 
150  illustrations.  Price  per  set:  Cloth,  $6.00  net;  Sheep  or  Half 
Morocco,  ^7.50  net. 

BY  ONE   OF  THE   GREATEST  OF  GERMAN   CLINICIANS 

This  book  is  a  new  one,  but  on  its  publication  it  sprang  into  immediate 
popularity,  and  is  now  one  of  the  leading  text-books  in  Germany.  It  is  prac- 
tically a  condensed  edition  of  the  author's  great  work  on  Special  Pathology  and 
Therapeutics,  and  it  forms  not  only  an  ideal  text-book  for  students,  but  a  practical 
guide  of  unusual  value  to  practising  physicians. 

Bulletin  of  Johns  Hopkins  Hospital 

"  This  book  is  an  excellent  one  of  its  kind.  Its  completeness,  yet  brevity,  the  clinical 
methods,  the  excellent  paragraphs  on  treatment  and  watering-places,  will  make  it  very  desir- 
able." 


Brid£[e  on  Tuberculosis 

Tuberculosis:  Recast  from  Lectures  Delivered  at  Rush  Medical 
College.  By  Norman  Bridge,  A.M.,  M.  D.,  Emeritus  Professor  of 
Medicine  in  Rush  Medical  College,  in  affiliation  with  the  University 
of  Chicago.     i2mo  of  302  pages,  illustrated.     Cloth,  ;^i.50  net. 

JUST   ISSUED 

In  this  excellent  work  the  practical  side  of  the  care  and  management  of  those 
sick  with  the  various  non-surgical  forms  of  tuberculosis  has  been  concisely  stated. 
Full  consideration  has  been  given  to  prophylaxis,  an  all-important  phase  of  the 
subject  that  has  heretofore  been  much  neglected.  There  are  also  chapters  upon 
the  Bacillus  of  Tuberculosis  ;  on  the  Pathology,  Etiology,  Symptoms,  Physical 
Signs,  Diagnosis,  and  Prognosis  of  the  disease,  each  treated  in  the  judicious  and 
thorough  manner  to  be  expected  in  a  work  by  such  a  well-known  authority  as  Dr. 
Bridge.     Treatment  is  accorded  unusual  space. 


SAUNDERS'   BOOKS   ON 


Sollmann*s  Pharmacology 

Including  Therapeutics,  Materia  Medica,  Pharmacy, 
Prescription -writing.  Toxicology,  etc. 


A  Text=Book  of  Pharmacology.  By  Torald  Sollmann,  M.  D., 
Assistant  Professor  of  Pharmacology  and  Materia  Medica,  Medical 
Department  of  Western  Reserve  University,  Cleveland,  Ohio.  Hand- 
some octavo  volume  of  894  pages,  fully  illustrated.     Cloth,  ^3.75  net. 

A   NEW   WORK— RECENTLY    ISSUED 

This  work  aims  to  furnish  a  scientific  discussion  and  definite  conception  of  the 
action  of  drugs,  as  well  as  their  derivation,  composition,  strength,  and  dose.  The 
author  bases  the  study  of  therapeutics  on  a  systematic  knowledge  of  the  nature 
and  properties  of  drugs,  and  thus  brings  out  forcibly  the  intimate  relation  between 
pharmacology  and  practical  medicine. 

J."  F.  Fotheringham,  M.  D. 

Prof,  of  Therapeutics  and  Theory  and  Practice  of  Prescribing,  Trinity  Med.  College,  Toronto. 
"The  work  certainly  occupies  ground  not  covered  in  so  concise,  useful,  and  scientific  a 
manner  by  any  other  text  I  have  read  on  the  subjects  embraced." 

Butler's   Materia   Medica 

Therapeutics,  and  Pharmacology 

A  Text=Bookof  Materia  Medica,  Therapeutics,  and  Pharmacology. 

By  George  F.  Butler,  Ph.  G.,  M.  D.,  Professor  of  Materia  Medica  and 
of  Clinical  Medicine,  College  of  Physicians  and  Surgeons,  Chicago, 
Octavo,  896  pages,  illustrated.  Cloth,  $4.00  net ;  Sheep  or  Half  Morocco, 
$5.00  net. 

FOURTH    EDITION,  REVISED   AND   ENLARGED 

In  this  new  edition  the  chapters  on  Organo-therapy,  Serum-therapy,  and  cog- 
nate subjects  have  been  enlarged  and  carefully  revised.  An  important  addition 
is  the  chapter  devoted  to  the  newer  theories  of  electrolytic  dissociation  and  its 
relation  to  the  topic  of  pharmacotherapy. 

Medical  Record,  New  York 

"  Nothing  has  been  omitted  by  the  author  which,  in  his  judgment,  would  add  to  the  com- 
pleteness of  the  text,  and  the  student  or  general  reader  is  given  the  benefit  of  latest  advices 
bearing  upon  the  value  of  drugs  and  remedies  considered." 


THE   PRACTICE    OF  MEDICINE.  13 


Gould   and   Pyie*s 
Curiosities  of  Medicine 


Anomalies  and  Curiosities  of  Medicine.  By  George  M.  Gould, 
M.  D.,  and  Walter  L.  Pyle,  M.  D.  An  encyclopedic  collection  of 
rare  and  extraordinary  cases  and  of  the  most  striking  instances  of 
abnormality  in  all  branches  of  Medicine  and  Surgery,  derived  from  an 
exhaustive  research  of  medical  literature  from  its  origin  to  the  present 
day,  abstracted,  classified,  annotated,  and  indexed.  Handsome  octavo 
volume  of  968  pages,  295  engravings,  and  12  full-page  plates. 

Popular  Edition  :  Cloth,  S3.OO  net  ;  Sheep  or  Half  Morocco,  $4-00  net. 

As  a  complete  and  authoritative  Book  of  Reference  this  work  will  be  of  value 
not  only  to  members  of  the  medical  profession,  but  to  all  persons  interested  in 
general  scientific,  sociologic,  and  medicolegal  topics  ;  in  fact,  the  absence  of  any 
complete  work  upon  the  subject  makes  this  volume  one  of  the  most  important 
literary  innovations  of  the  day. 

The  Lancet,  London 

"The  book  is  a  monument  of  untiring  energy,  keen  discrimination,  and  erudition.  .  .  . 
We  heartily  recommend  it  to  the  profession." 

Saunders*  Year-Book 


The  American  Year=Book  of  Medicine  and  Surgery.  A  Yearly 
Digest  of  Scientific  Progress  and  Authoritative  Opinion  in  all  branches 
of  Medicine  and  Surgery,  drawn  from  journals,  monographs,  and  text- 
books of  the  leading  American  and  foreign  authors  and  investigators. 
Arranged  with  critical  editorial  comments  by  eminent  American  special- 
ists, under  the  editorial  charge  of  George  M.  Gould,  M.  D.  In  two 
volumes — Vol.  I.,  including  General  Medicine ;  Vol.  II.,  General  Sur- 
gery. Per  volume  :  Cloth,  ;^3. 00  net;  Half  Morocco,  ^3.75  net.  Sold 
by  Subscription. 

EQUIVALENT  TO  A   POSTGRADUATE  COURSE 

The  Lancet.  London 

"  It  is  much  more  than  a  mere  compilation  of  abstracts,  for,  as  each  section  is  entrusted  to 
experienced  and  able  contributors,  the  reader  has  the  advantage  of  certain  critical  commen- 
taries and  expositions  .  .  proceeding  from  writers  fully  qualified  to  perform  these  tasks." 


14  SAUNDERS'   BOOKS    ON 

Thornton's   Dose-Book 

Dose=Book  and  Manual  of  Prescription=Writing.  By  E.  Q.  Thorn- 
ton, M.  D.,  Assistant  Professor  of  Materia  Medica,  Jefferson  Medical 
College,  Phila.  Post-octavo,  362  pages,  illustrated.  Flexible  Leather, 
$2.00  net. 

Second  Edition,  Revised  and  Enlarged 

In  the  new  edition  of  this  work,  intended  for  the  student  and  practitioner, 
additions  have  been  made  to  the  chapters  on  "Prescription-Writing"  and 
"  IncompatibiHties,"  and  references  have  been  introduced  in  the  text  to  the 
newer  curative  sera,  organic  extracts,  synthetic  compounds,  and  vegetable  drugs. 
To  the  Appendix,  chapters  upon  Synonyms  and  Poisons  and  their  antidotes 
have  been  added,  thus  increasing  its  value  as  a  book  of  reference. 

C.  H.  Miller.  M.  D., 

Professor  of  Pharmacology,  North-western  University  Medical  School,  Chicago. 

"  I  will  be  able  to  make  considerable  use  of  that  part  of  its  contents  relating  to  the  correct 
terminology  as  used  in  prescription-writing,  and  it  will  afford  me  much  pleasure  to  recommend 
the  book  to  my  classes,  who  often  fail  to  find  this  information  in  their  other  text-books." 


American  Text-Book  of 
Applied  Therapeutics 

American  Text=Book  of  Applied  Therapeutics.  Edited  by  James 
C.  Wilson,  M.  D.,  Professor  of  Practice  of  Medicine  and  of  Clinical 
Medicine,  Jefferson  Medical  College,  Philadelphia.  Handsome  imperial 
octavo  volume  of  1326  pages.  Illustrated.  Cloth,  ^7.00  net;  Sheep 
or  Half  Morocco,  $8.00  net. 

For  Student  and  Practitioner 

Written  for  both  the  student  and  practitioner,  the  aim  of  this  work  is  to 
facilitate  the  apphcation  of  knowledge  to  the  prevention,  cure,  and  alleviation 
of  disease.  The  endeavor  throughout  has  been  to  conform  to  the  title  of  the 
book — "Applied  Therapeutics" — to  indicate  the  course  of  treatment  to  be  pur- 
sued at  the  bedside,  rather  than  to  name  a  list  of  drugs  that  have  been  used  at 
one  time  or  another.      The  work  will  be  found  accurate  and  trustworthy. 

Buffalo  Medical  Journal 

"  It  is  one  of  the  most  complete  books  of  reference  that  has  been  presented  to  the  profes- 
sion on  medicine  in  a  long  period  of  time  ;  and  never  before  have  we  had  one  that  undertook 
to  cover  the  field  in  this  manner." 


PRACTICE,   MATERIA    MEDIC  A,   Etc.  15 

The  American  Pocket  Medical  Dictionary.  Fourth  Edition,  Revised 

The  American  Pocket  Medical  Dictionary.  Edited  by  W.  A.  Newman  Dor- 
land,  M.  D.,  Assistant  Obstetrician  to  the  Hospital  of  the  University  of  Pennsylvania. 
Containing  the  pronunciation  and  definition  of  the  principal  words  used  in  medicine 
and  kindred  sciences,  with  64  extensive  tables.  Flexible  leather,  with  gold  edges, 
^l.oo  net ;  with  thumb  index,  $1.25  net. 

"I  can  recommend  it  to  our  students  without  reserve." — J.  H.  Holland,  M.  D.,  Dean  of  the 
Jefferson  Medical  College,  Philadelphia. 

Vierordt's    Medical    Diag^nosis.      Fourth  Edition,  Revised 

Medical  Diagnosis.  By  Dr.  Oswald  Vierordt,  Professor  of  Medicine,  Univer- 
sity of  Heidelberg.  Translated  from  the  fifth  enlarged  German  edition  by  Francis 
H.  Stuart,  A.  M. ,  M.  D.  Octavo,  603  pages,  104  wood  cuts.  Cloth,  ^4.00  net; 
Sheep  or  Half  Morocco,  ^5.00  net. 

"  Has  been  recognized  as  a  practical  work  of  the  highest  value.  It  may  be  considered  indispensable 
both  to  students  and  practitioners." — F.  Minot,  M.  D.,  late  Professor  of  Theory  and  Practice  in 
Harvard  University. 

Cohen   and    Cshner's   Dia£>nOsis.      Second  Revised  Edition 

Essentials  of  Diagnosis.  By  S.  Solis-Cohen,  M.  D.,  Senior  Assistant  Professor 
in  Clinical  Medicine,  Jefferson  Medical  College,  Phila.  ;  and  A.  A.  Eshner,  M.  D., 
Professor  of  Clinical  Medicine,  Philadelphia  Polyclinic.  Post-octavo,  382  pages  ;  55 
illustrations.      Cloth,  $1.00  net.     In  Sautiders'  Question- Compend  Series. 

"  Concise  in  the  treatment  of  subject,  terse  in  expression  of  fact." — American  Journal  of  the 
Medical  Sciences. 

Morris*  Materia  Medica  and  Therapeutics.     Fifth  Revised  Edition 

Essentials  of  Materia  Medica,  Therapeutics,  and  Prescription-Writing. 
By  Henry  Morris,  M.  D.,  late  Demonstrator  of  Therapeutics,  Jefferson  Medical 
College,  Phila.  Post-octavo,  250  pages.  Cloth,  ^i.co  net.  In  Saunders'  Question- 
Compend  Series. 

"  Cannot  fail  to  impress  the  mind  and  instinct  in  a  lasting  manner." — Buffalo  Medical  Journal. 

Sayre's  Practice  of  Pharmacy.     Second  Edition.  Revised 

Essentials  of  the  Practice  of  Pharmacy.  By  Lucius  E.  Sayre,  M.  D.,  Pro- 
fessor of  Pharmacy,  University  of  Kansas.  Post-octavo,  200  pages.  Cloth,  ^i. 00  net. 
In  Saunders''  Question- Compend  Series. 

"  The  topics  are  treated  in  a  simple,  practical  manner,  and  the  work  forms  a  very  useful  student's 
manual." — Boston  Medical  and  Surreal  Journal. 

Brockway's   Medical    Physics.      Second  Edition.  Revised 

Essentials  of  Medical  Physics.  By  Fred.  J.  Brockway,  M.  D.,  late  Assistant 
Demonstrator  of  Anatomy,  College  of  Physicians  and  Snrgeons,  N.  Y.  Post-octavo, 
330  pages  ;  155  fine  illustrations.  Cloth,  $l.OD  net.  In  Saunders^  Question- Co^npend 
Series. 

"  It  contains  all  that  one  need  know  on  the  subject,  is  well  written,  and  is  copiously  illustrated." — 
Medical  Record,  New  York. 

Stoney's  Materia  Medica  for  Nurses 

Materia  Medica  for  Nurses.  By  Emily  A.  M.  Stoney,  Superintendent  of  the 
Training  School  for  Nurses  at  the  Carney  Hospital,  South  Boston,  Mass.  Handsome 
octavo  volume  of  306  pages.      Cloth,  $1.50  net. 

"  It  contains  about  everj-thing  that  a  nurse  ought  to  know  in  regard  to  drags." ^Journal  of  the 
American  Medical  Association. 

Grafstrom's  Mechano-therapy 

A  Text-Book  of  Mechano-therapy  (Massage  and  Medical  Gymnastics).  By 
Axel  V.  Grafstrom,  B.  Sc,  M.  D.,  late  House  Physician.  City  Hospital,  Black- 
well's  Island,  N.  Y.      l2mo,  139  pages,  illustrated.     $1.00  net. 

"Certainly  fulfills  its  mission  in  rendering  comprehensible  the  subjects  of  massage  and  medical 
gymnastics." — Ne'w   York  Medical  Journal. 


1 6  SAUNDERS'    BOOKS    ON  PRACTICE,   Etc. 

Jakob  and  Eshner's  Internal  Medicine  and  Diagnosis 

Atlas  and  Epitome  of  Internal  Medicine  and  Clinical  Diagnosis.  By  Dr. 
Chk.  Jakob,  of  Erlangen.  Edited,  with  additions,  by  A.  A.  Eshner,  M.  D.,  Pro- 
fessor of  Clinical  Medicine,  Philadelphia  Polyclinic.  With  182  colored  figures  on 
68  plates,  64  text-illustrations,  259  pages  of  text.  Cloth,  ^3.00  net.  In  Saunders^ 
Hand-Atlas  Series. 

"  Can  be  recommended  unhesitatingly  to  tlie  practicing  physician  no  less  than  to  the  student." — 
Bulletin  0/ Johns  Hopkins  Hospital. 

Lockwood's    Practice    of    Medicine.  Revised  and^Enlarged 

A  Manual  of  the  Practice  of  Medicine.  By  Geo.  Roe  Lockwood,  M.  D., 
Attending  Physician  to  the  Bellevue  Hospital,  New  York  City.  Octavo,  847  pages, 
with  79  illustrations  in  the  text  and  22  full-page  plates.      Cloth,  ^4.00  net. 

"  A  work  of  positive  merit,  and  one  which  we  gladly  welcome." — New  York  Medical  Journal. 

Salinger  and  Kalteyer's  Modern  Medicine 

Modern  Medicine.  By  Julius  L.  Salinger,  M.  D.,  late  Demonstratoi  of  Clinical 
Medicine,  Jefferson  Medical  College;  and  F.  J.  Kalteyer,  M.D.,  Demonstrator  of 
Clinical  Medicine,  Jefferson  Medical  College.  Handsome  octavo,  801  pages,  illus- 
trated.    Cloth,  ^4.00  net. 

"  I  have  carefully  examined  the  book,  and  find  it  to  be  thoroughly  trustworthy  in  all  respects  and  a 
valuable  text-book  for  the  medical  student." — Sam'l  O.  L.  Potter,  Formerly  Professor  of  Principles 
and  Practice  of  Medicine,  Cooper  Medical  College,  San  Francisco. 

Keating's  Life  Insurance 

How  to  Examine  for  Life  Insurance.  By  the  late  John  M.  Keating,  M.  D., 
Ex-President  of  the  Association  of  Life  Insurance  Medical  Directors.  Royal  octavo, 
211  pages.     With  numerous  illustrations.      Cloth,  $2.00  net. 

"  This  is  by  far  the  most  useful  book  which  has  yet  appeared  on  insurance  examination." — Medical 
News. 

Corwin's   Physical    Diagnosis.      Third  Edition,  Revised 

Essentials  of  Physical  Diagnosis  of  the  Thorax.  By  A.  M.  Corwin,  A.  M., 
M.  D.,  Instructor  of  Physical  Diagnosis  in  Rush  Medical  College,  Chicago.  220 
pages,  illustrated.      Cloth,  flexible  covers,  $1.25  net. 

"  A  most  excellent  little  work.  It  arranges  orderly  and  in  sequence  the  various  objective  phenomena 
to  logical  solution  of  a  careful  diagnosis." — Journal  of  Nervous  and  Mental  Diseases. 

American  Text-Book  of  Theory  and  Practice 

American  Text-Book  of  the  Theory  and  Practice  of  Medicine.  Edited 
by  the  late  William  Pepper,  M.  D.,  LL.  D.,  Professor  of  the  Theory-  and  Practice 
of  Medicine  and  of  Clinical  Medicine,  University  of  Penna.  Two  handsome  imperial 
octavos  of  about  1000  pages  each.  Illustrated.  Per  volume  :  Cloth,  ^5.00  net ;  Sheep 
or  Half  Morocco,  ^6.00  net. 

■'  I  am  quite  sure  it  will  command  itself  both  to  practitioners  and  students  of  medicine,  and  become 
one  of  our  most  popular  text-books." — Alfred  LiOomis,  M.  D.,  LL.  D.,  Professor  of  Pathology  and 
Practice  of  Medicine,  University  of  the   City  of  New   York. 

Stevens*  Practice  of   Medicine.      Sixth  Edition,  Revised— just  issued 

A  Manual  of  the  Practice  of  Medicine.     By  A.  A.  Stevens,  A.  M.,  M.  D., 
Lecturer  on  Physical   Diagnosis,  University  of  Pennsylvania.      Specially  intended  for 
.  students  preparing  for  graduation  and  hospital  examinations.      Post-octavo,  519  pages  ; 
illustrated.     Flexible  leather,  ^2.00  net. 

"  An  excellent  condensation  of  the  essentials  of  medical  practice  for  the  student,  and  may  be  found 
also  an  excellent  reminder  for  the  busy  physician." — Buffalo  Medical  Journal. 


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